Abstract: The duplicate gear and its coupled 512 gear of a gear box both failed during bench test. The methods of fracture analysis, metallographic examination, hardness test and chemical compositions analysis were used to test the failure parts. The result show that the serious abnormal contact surface between the duplicate gear and 512 gear, as well as the off-specification of hardening depth and core hardness of the duplicate gear caused severe contact fatigue spalling on surface of the duplicate gear and bending fatigue fracture of the 512 gear.