diff --git a/ppdet/data/transform/gridmask_utils.py b/ppdet/data/transform/gridmask_utils.py
index 2b3e72408c24f0b32559eb30cc8a451fecdb9d00..f0775dc3177b166b1a50ff4704b2d1edb0315ea2 100644
--- a/ppdet/data/transform/gridmask_utils.py
+++ b/ppdet/data/transform/gridmask_utils.py
@@ -45,7 +45,8 @@ class Gridmask(object):
         self.prob = self.st_prob * min(1, 1.0 * curr_iter / self.upper_iter)
         if np.random.rand() > self.prob:
             return x
-        h, w, _ = x.shape
+        # image should be C, H, W format
+        _, h, w = x.shape
         hh = int(1.5 * h)
         ww = int(1.5 * w)
         d = np.random.randint(2, h)
diff --git a/static/ppdet/data/transform/gridmask_utils.py b/static/ppdet/data/transform/gridmask_utils.py
index 115cb1e9d291be365ce0436373c9e5eac00acb4c..b370bf0078a3f960fdf0d65cc7057153eff49b68 100644
--- a/static/ppdet/data/transform/gridmask_utils.py
+++ b/static/ppdet/data/transform/gridmask_utils.py
@@ -45,7 +45,8 @@ class GridMask(object):
         self.prob = self.st_prob * min(1, 1.0 * curr_iter / self.upper_iter)
         if np.random.rand() > self.prob:
             return x
-        h, w, _ = x.shape
+        # image should be C, H, W format
+        _, h, w = x.shape
         hh = int(1.5 * h)
         ww = int(1.5 * w)
         d = np.random.randint(2, h)