diff --git a/utils/utils.py b/utils/utils.py
index 1ded9eae..0af7e5c4 100755
--- a/utils/utils.py
+++ b/utils/utils.py
@@ -272,9 +272,11 @@ def bbox_iou(box1, box2, x1y1x2y2=True, GIoU=False, DIoU=False, CIoU=False):
             rho2 = ((b2_x1 + b2_x2) - (b1_x1 + b1_x2)) ** 2 / 4 + ((b2_y1 + b2_y2) - (b1_y1 + b1_y2)) ** 2 / 4
             if DIoU:
                 return iou - rho2 / c2  # DIoU
-            elif CIoU:
+            elif CIoU:  # https://github.com/Zzh-tju/DIoU-SSD-pytorch/blob/master/utils/box/box_utils.py#L47
                 v = (4 / math.pi ** 2) * torch.pow(torch.atan(w2 / h2) - torch.atan(w1 / h1), 2)
-                return iou - (rho2 / c2 + v ** 2 / (1 - iou + v))  # CIoU
+                with torch.no_grad():
+                    alpha = v / (1 - iou + v)
+                return iou - (rho2 / c2 + v * alpha)  # CIoU
 
     return iou