Abstract: A great number of inner layer flaw alarms are triggered with the Φ32 mm×2 mm TP316L seamless steel pipes as being ultrasonic detected,and the conformance rate is just 60%. In order to identify the causes for the said alarms,firstly the pipe is subjected to dye detection,and no crack is found on both outer and inner surfaces thereof;and then the metallographic test sample is taken after the defect is located. As a result of the analysis it is concluded that the above mentioned nonconformance is attributed to the lamellar nonmetallic inclusions inside the steel pipe which are mainly the silicate and alumina inclusions.