The morphologies of failures in synchronous belts were observed following four types of fatigue test. Two modes of initiation were observed. One was due to cord failure which was observed under various conditions. Detailed observations showed that the interfaces between stranded glass filaments were separated at the centre of the cords. The extended direction of the separations was mainly vertical to the back face of the synchronous belt. The second mode of crack initiation was observed under relatively high loadings of the mating flank of the belt tooth. The cracks were initiated at the tooth roots of the synchronous belts. Finally, the deformation behaviour of a synchronous belt was measured under static loading.