diff --git a/pygmt/tests/test_grdfilter.py b/pygmt/tests/test_grdfilter.py
index 7d87dc6fe51..d86add635f0 100644
--- a/pygmt/tests/test_grdfilter.py
+++ b/pygmt/tests/test_grdfilter.py
@@ -45,7 +45,7 @@ def test_grdfilter_dataarray_in_dataarray_out(grid, expected_grid):
     Test grdfilter with an input DataArray, and output as DataArray.
     """
     result = grdfilter(
-        grid=grid, filter="g600", distance="4", region=[-53, -49, -20, -17]
+        grid=grid, filter="g600", distance="4", region=[-53, -49, -20, -17], cores=2
     )
     # check information of the output grid
     assert isinstance(result, xr.DataArray)
diff --git a/pygmt/tests/test_grdsample.py b/pygmt/tests/test_grdsample.py
index 06897d1ab28..a30a6a248e4 100644
--- a/pygmt/tests/test_grdsample.py
+++ b/pygmt/tests/test_grdsample.py
@@ -74,7 +74,7 @@ def test_grdsample_dataarray_out(grid, expected_grid, region, spacing):
     """
     Test grdsample with no outgrid set and the spacing is changed.
     """
-    result = grdsample(grid=grid, spacing=spacing, region=region)
+    result = grdsample(grid=grid, spacing=spacing, region=region, cores=2)
     # check information of the output grid
     assert isinstance(result, xr.DataArray)
     assert result.gmt.gtype == 1  # Geographic grid
diff --git a/pygmt/tests/test_sph2grd.py b/pygmt/tests/test_sph2grd.py
index 1876be96454..aed75d1c155 100644
--- a/pygmt/tests/test_sph2grd.py
+++ b/pygmt/tests/test_sph2grd.py
@@ -26,7 +26,7 @@ def test_sph2grd_no_outgrid():
     """
     Test sph2grd with no set outgrid.
     """
-    temp_grid = sph2grd(data="@EGM96_to_36.txt", spacing=1, region="g")
+    temp_grid = sph2grd(data="@EGM96_to_36.txt", spacing=1, region="g", cores=2)
     assert temp_grid.dims == ("y", "x")
     assert temp_grid.gmt.gtype == 0  # Cartesian grid
     assert temp_grid.gmt.registration == 0  # Gridline registration