Abstract: When using an ultrasonic thickness gauge to measure the steel pipe,sometimes the indication values resulting from the testing are less than the real ones,and in such cases,the pipes being measured are always found to have inner bulges and fins.Classified statistics and sample analysis are made to such cases.As a result,it is identified that the problem concerning smaller indication values is caused by the nonmetallic inclusions as distributed in the middle wall area and the area close to the inner surface of the pipe,while the bulges and fins of the pipe are caused by the nonmetallic inclusions as held in the area just under the inner surface thereof.The said inner fin of the pipe is substantially different from the inner fold of the shell as occurring due to the deformation produced by the 2-roll rotary piercing process.