Abstract: A system analysis had been conducted on the end reduction input gearwheel of an engine to find its fatigue failure causes, and the failure process of the gearwheel was confirmed by means of macro inspection, fracture analysis, metallographic examination, stress analysis, etc. Fatigue crack initiated at the root of the central part of the bevel gearwheel under big bending loads. In the beginning, crack extended along the radial direction in fatigue way with a small angle about 15°. The appearance of crack caused the variation of gearwheel vibration frequency, so pitch diameter type vibration appeared in the gearwheel. At last the gearwheel crack extended along the radial direction in fatigue way. After the gearwheel crack of radial direction, reduction of the cross-section and low intensity because of there being almost no infiltration layer in the starting site caused remaining part of the gearwheel to appear a secondary bending fatigue fracture.