Thanks your advice. But I still didn’t understand about that why MaskRcnn with ResNet backbone is okay but that with ResNext is invalid. This Nan always occurs with any dataset including COCO2017 when ResNext or Wide-Resnet is used as backbone. I just got the backbone by using   vision/torchvision/models/detection/backbone_utils.py, and changed the default Mask Rcnn provided from torchvision 1. Is this a possible situation?