Abstract: The intensive flaws were found in a batch of 42CrMo steel hub forgings when ultrasonic inspecting, which resulted in the scrap of the forgings. By means of macrostructure examination, metallographic examination and scanning electron microscope analysis, the properties and causes of the ultrasonic inspection flaws of the hub forgings were analyzed. The results show that the ultrasonic inspection flaws were flake cracks which formed under the coactions of hydrogen and normalizing cooling stress. It was suggested that the forgings should avoid direct air cooling after forging, and should be treated by dehydrogenation heat treatment too.