Mechanics of cords under bending is important to understand the fatigue failure mechanism of synchronous belts, because the fatigue damage is usually accumulated in the cords when the belt works. In this paper, we discussed the mechanical meaning of curvature in cord-bending. At first, the distributions of stresses which were initiated in cords were discussed by using FEM analysis. The FEM model of cords was constructed from seven fibers and elastomer layer between the fibers. The results showed that the shearing stress along the fiber-axis was highest in the elastomer layer and had the maximum value when the layer turned to the vertical direction against the neutral a is of the cord-bending. These results suggested us that the shearing stress was the main stress component for the cord failure. We then discussed the relationship between the shearing stress and the curvature by using a simplified mechanical model. The analytical result reasonably explained the FEM results and showed that the shearing stress increased in exact proportion to the curvature.