Fatigue failure of synchronous belts is investigated under fluctuating torque conditions, because the driving torque fluctuates in the automotive engine camshaft drive-systems. There are two types of fatigue-failure modes. The first mode is de-bonding along the interfaces between the stranded glass filaments in cords. The initiation of the de-bonding is explained by the cyclic bending deformation when the belt is wound around pulleys and transmits the power. A simplified mechanical model is proposed to explain the failure mechanism. The second mode is crack initiation between the cords and the facing fabrics. Further investigation is necessary to reveal the second failure mode.