Abstract: After factory test, two fluorescent display linetype defects were found on the end face of a 12Cr2Ni4A gear used for an aero engine. Techniques of macro and electron microscopy check, metallographic examination, chemical analysis and simulation testing were adopted to determine the nature and cause of the defects. The results show that one of the defects was a scratch and another was a thermal stress crack which was initiated in the machining grinding process due to the existence of large residual stress under work stress. Some appropriate corrective measures are suggested. Controlling the amount of feed in the machining and shortening the tempering time interval after grinding may effectively reduce the probability of occurrence of cracking.