The understanding of cord failure mechanism is important for the strengthening and the fatigue life prediction in synchronous belts because the failure usually initiate and accumulate in the cords. Firstly, we observed the fatigue failure morphology of belt-cords and confirmed that the cords were damaged at the interface between stranded glass filaments. Then, the stress field between the strands in cords were analyzed by using FEM analysis. The results showed that the shear stress along the strand-axis was considered to cause the fatigue failure because the amount of it was highest and the direction of it was identical with the observed failure morphology. Finally, the shear stress was analyzed by using a simplified mechanical model to obtain an equation to express the property of the shear stress. The analytical results were qualitatively the same as the FEM results, and showed that the curvature was a good parameter to estimate the failure of cords.