Reduction of bending stiffness in helical cords during bending fatigue was investigated carrying out some experiments, FEM analysis and mechanical analysis. The cords were constructed of stranded glass filaments and Resorcinol-Formalin Latex, and were used in synchronous belts. The stranded glass filaments were de-bonded and the bending stiffness decreased after the bending-fatigue. The FEM analysis showed that the reduction could be explained by the initiation and extension of the de-bonding in the cords. A simplified mechanical model was proposed to explain the reduction of bending stiffness of cords.