Abstract: Gears of the constant speed drive used in aircraft power system failed after running for 124 hours.For finding the failure reason macro and micro analysis on the fractured gears were performed by means of stereo microscopy,SEM and EDX.The carburized depth of the gear tooth was examined by hardness testing.The results show that early fatigue fracure of the differential gear happened first.Large size aluminium oxide inclusions at the teeth root of the differential gear resulted in stress concentration and formation of crack source.Effective hardening didnt form at the teeth root,resulting in the decrease of the fatigue strength greatly.So the fatigue crack grew and led to the final fracture of the differential gear.Then overload breakage of the input gear occured.