diff --git a/pygmt/datatypes/header.py b/pygmt/datatypes/header.py
index 04e10ac0c72..ab109521131 100644
--- a/pygmt/datatypes/header.py
+++ b/pygmt/datatypes/header.py
@@ -203,7 +203,8 @@ def data_attrs(self) -> dict[str, Any]:
         Attributes for the data variable from the grid header.
         """
         attrs: dict[str, Any] = {}
-        attrs["Conventions"] = "CF-1.7"
+        if self.type == 18:  # Grid file format: ns = GMT netCDF format
+            attrs["Conventions"] = "CF-1.7"
         attrs["title"] = self.title.decode()
         attrs["history"] = self.command.decode()
         attrs["description"] = self.remark.decode()