diff --git a/CHANGES.rst b/CHANGES.rst
index 580c6ed..b32e169 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -5,6 +5,7 @@ Changelog
 ------
 
 - Fix ``enctype`` attribute setting for WTForms ``MultipleFileField``.
+- Fix field class append bug when using ``render_kw={'class': 'my-class'}`` (`#53 <https://github.com/greyli/bootstrap-flask/issues/53>`__).
 
 1.2.0
 ------
diff --git a/flask_bootstrap/templates/bootstrap/form.html b/flask_bootstrap/templates/bootstrap/form.html
index d0c1a6d..8cc2207 100644
--- a/flask_bootstrap/templates/bootstrap/form.html
+++ b/flask_bootstrap/templates/bootstrap/form.html
@@ -45,7 +45,7 @@
         {% call _hz_form_wrap(horizontal_columns, form_type, True, required=required) %}
             <div class="form-group form-check{% if form_type == "inline" %} form-check-inline{% endif %}">
                 <label class="form-check-label">
-                    {{ field(class_="form-check-input")|safe }} {{ field.label.text|safe }}
+                    {{ field(class_="form-check-input %s" % field.render_kw.class)|safe }} {{ field.label.text|safe }}
                 </label>
             </div>
         {% endcall %}
@@ -54,12 +54,12 @@
      this is just a hack for now, until I can think of something better #}
         <div class="form-group {% if form_type == 'horizontal' %}row{% endif %}{% if required %} required{% endif %}">
             {%- if form_type == "inline" %}
-                {{ field.label(class="sr-only")|safe }}
+                {{ field.label(class="sr-only %s" % field.render_kw.class)|safe }}
             {% elif form_type == "horizontal" %}
                 {{ field.label(class="form-control-label " + (
-                " col-%s-%s" % horizontal_columns[0:2]))|safe }}
+                " col-%s-%s %s" % (horizontal_columns[0:2], field.render_kw.class)))|safe }}
             {%- else -%}
-                {{ field.label(class="form-control-label")|safe }}
+                {{ field.label(class="form-control-label %s" % field.render_kw.class)|safe }}
             {% endif %}
             {% if form_type == 'horizontal' %}
               <div class=" col-{{ horizontal_columns[0] }}-{{ horizontal_columns[2] }}">
@@ -83,7 +83,7 @@
 
         {# note: same issue as above - should check widget, not field type #}
         {% call _hz_form_wrap(horizontal_columns, form_type, True, required=required) %}
-            {{ field(class='btn btn-%s' % button_map.get(field.name, 'secondary'),
+            {{ field(class='btn btn-%s %s' % (button_map.get(field.name, 'secondary'), field.render_kw.class),
             **field_kwargs) }}
         {% endcall %}
     {%- elif field.type in ['FormField', 'FieldList'] -%}
@@ -109,15 +109,15 @@
                 {{ field.label(class="sr-only")|safe }}
                 {% if field.type == 'FileField' %}
                     {% if field.errors %}
-                        {{ field(class="form-control-file is-invalid", **kwargs)|safe }}
+                        {{ field(class="form-control-file is-invalid %s" % field.render_kw.class, **kwargs)|safe }}
                     {% else %}
-                        {{ field(class="form-control-file", **kwargs)|safe }}
+                        {{ field(class="form-control-file %s" % field.render_kw.class, **kwargs)|safe }}
                     {% endif %}
                 {% else %}
                     {% if field.errors %}
-                        {{ field(class="form-control mb-2 mr-sm-2 mb-sm-0 is-invalid", **kwargs)|safe }}
+                        {{ field(class="form-control mb-2 mr-sm-2 mb-sm-0 is-invalid %s" % field.render_kw.class, **kwargs)|safe }}
                     {% else %}
-                        {{ field(class="form-control mb-2 mr-sm-2 mb-sm-0", **kwargs)|safe }}
+                        {{ field(class="form-control mb-2 mr-sm-2 mb-sm-0 %s" % field.render_kw.class, **kwargs)|safe }}
                     {% endif %}
                 {% endif %}
             {% elif form_type == "horizontal" %}
@@ -125,15 +125,15 @@
                 <div class=" col-{{ horizontal_columns[0] }}-{{ horizontal_columns[2] }}">
                     {% if field.type == 'FileField' %}
                         {% if field.errors %}
-                            {{ field(class="form-control-file is-invalid", **kwargs)|safe }}
+                            {{ field(class="form-control-file is-invalid %s" % field.render_kw.class, **kwargs)|safe }}
                         {% else %}
-                            {{ field(class="form-control-file", **kwargs)|safe }}
+                            {{ field(class="form-control-file %s" % field.render_kw.class, **kwargs)|safe }}
                         {% endif %}
                     {% else %}
                         {% if field.errors %}
-                            {{ field(class="form-control is-invalid", **kwargs)|safe }}
+                            {{ field(class="form-control is-invalid %s" % field.render_kw.class, **kwargs)|safe }}
                         {% else %}
-                            {{ field(class="form-control", **kwargs)|safe }}
+                            {{ field(class="form-control %s" % field.render_kw.class, **kwargs)|safe }}
                         {% endif %}
                     {% endif %}
                 </div>
@@ -152,15 +152,15 @@
                 {{ field.label(class="form-control-label")|safe }}
                 {% if field.type == 'FileField' %}
                     {% if field.errors %}
-                        {{ field(class="form-control-file is-invalid", **kwargs)|safe }}
+                        {{ field(class="form-control-file is-invalid %s" % field.render_kw.class, **kwargs)|safe }}
                     {% else %}
-                        {{ field(class="form-control-file", **kwargs)|safe }}
+                        {{ field(class="form-control-file %s" % field.render_kw.class, **kwargs)|safe }}
                     {% endif %}
                 {% else %}
                     {% if field.errors %}
-                        {{ field(class="form-control is-invalid", **kwargs)|safe }}
+                        {{ field(class="form-control is-invalid %s" % field.render_kw.class, **kwargs)|safe }}
                     {% else %}
-                        {{ field(class="form-control", **kwargs)|safe }}
+                        {{ field(class="form-control %s" % field.render_kw.class, **kwargs)|safe }}
                     {% endif %}
                 {% endif %}
                 {%- if field.errors %}
diff --git a/test_bootstrap_flask.py b/test_bootstrap_flask.py
index 39ea015..c20bd4b 100644
--- a/test_bootstrap_flask.py
+++ b/test_bootstrap_flask.py
@@ -98,8 +98,8 @@ def test():
 
         response = self.client.get('/field')
         data = response.get_data(as_text=True)
-        self.assertIn('<input class="form-control" id="username" name="username"', data)
-        self.assertIn('<input class="form-control" id="password" name="password"', data)
+        self.assertIn('<input class="form-control " id="username" name="username"', data)
+        self.assertIn('<input class="form-control " id="password" name="password"', data)
 
     def test_render_form(self):
         @self.app.route('/form')
@@ -112,8 +112,8 @@ def test():
 
         response = self.client.get('/form')
         data = response.get_data(as_text=True)
-        self.assertIn('<input class="form-control" id="username" name="username"', data)
-        self.assertIn('<input class="form-control" id="password" name="password"', data)
+        self.assertIn('<input class="form-control " id="username" name="username"', data)
+        self.assertIn('<input class="form-control " id="password" name="password"', data)
 
     def test_render_form_row(self):
         @self.app.route('/form')
@@ -360,3 +360,22 @@ def multi():
         response = self.client.get('/multi')
         data = response.get_data(as_text=True)
         self.assertIn('multipart/form-data', data)
+
+
+    # test WTForm fields for render_form and render_field
+    def test_form_render_kw_class(self):
+        class TestForm(FlaskForm):
+            submit = SubmitField(render_kw={'class': 'my-awesome-class'})
+
+        @self.app.route('/render_kw')
+        def render_kw():
+            form = TestForm()
+            return render_template_string('''
+            {% from 'bootstrap/form.html' import render_form %}
+            {{ render_form(form) }}
+            ''', form=form)
+
+        response = self.client.get('/render_kw')
+        data = response.get_data(as_text=True)
+        self.assertIn('my-awesome-class', data)
+        self.assertIn('btn', data)