Dear @ptrblck  thanks for your interest. I wander if the test loss behaviour comes from the BN or if it is a pb with resnets model for my images…
During the training I use model.train(), during the testing I use model.eval() and the model is not change between the two phases that I perform at each epoch (same batch size= 128 for train & test)
When I switch OFF complety the transformations of the train set, and uses the same set for train & test, I got the same behviour:

And finaly, if I switch off the suffling and random transforms (flips & Rotations) of train set, and I use the same set for test, then I get:

Seems that the test loss is converging towards a value, but different from the train loss.