Abstract: The defects of forging gear drive shaft(hereinafter referred to as gear shaft) often lie in the centre of shaft and it is easy to find these internal defects by ultrasound. There is however no other useful auxiliary test method to test these internal defects, so it is difficult to ascertain the character of defects. This article determines the character of these defects by ultrasonic test and dissection analysis, and reduces the risk of use of this gear shaft with harmful defects in it.