Fatigue failure mechanism in synchronous belts with steel cord was investigated. The steel cord was made of twisting nineteen steel wires. The construction of the cord is described as 1  19, which consists of one core wire, six inner layer wires and twelve outer layer wires. The belt failure was initiated near the tooth roof, where the steel cords were fatigued. The curvature of the belt was measured and confirmed that the belt was bent largely at the failure initiation site. Namely, the belt was damaged by the bending fatigue. Then, the bending fatigue tests of the steel cord were carried out to examine the fatigue failure mechanism. The fatigue failure was firstly initiated in the outer layer wires. However, the bending stress was lowest in the outer layer wires and is highest in the core wire, according to a simple geometrical analysis. Therefore, it is necessary to consider the initiation caused another type pf stresses based on the contact between the wires.