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Fatigue failure under fluctuating torque is important because the driving torque fluctuates in automotive engine camshaft drive-systems. The authors observed that the fatigue failure initiated at the tooth root on the opposite side of working flank under fluctuating torque conditions, while the failure initiated at the tooth root on the working flank side under constant torque conditions. In this study, the main factor that decides the failure initiation site is discussed, caring out some experiments and finite element analysis (FEA). According to the FEA results, the maximum stress initiation site is moved from the opposite side of working flank to the working flank side, increasing with the tooth load. The main factor that decides the failure initiation site is the distribution of tooth load on the working flank. The tooth load distributed near the tooth tip induces the high shearing stress at the tooth root on the opposite side of working flank, while the distributed load near the tooth root induces the high shearing stress at the tooth root on the working flank side.