Abstract: The internal defects were found by ultrasonic testing in manufacturing process of 34CrMo1A rotor. The internal flaw type of rotor was analyzed by means of macroscopic examination, metallographic examination, chemical composition analysis, fracture analysis and so on. The results showed that the type of internal defects was flake, whose micro fractography showed as radial stripes typical characteristics of flake, and alumina inclusions were also found in the rotor. The high content of residual hydrogen in rotor was the main reason of flake defect.