Considering the flexural rigidity, sagging and geometric nonlinearity, the nonlinear responses of stay cables in cablestayed beams were investigated. The nonlinear dynamical equations of the cablestayed beam were formulated by Hamiltonian principle. Then, these equations were discretized by the Galerkin method, and the responses of the cable in the cablestayed beam were obtained using the Multiple Scales method. The influence of main parameters on the fundamental frequency of the cable in plane was analyzed. The effects of flexural rigidity on both the frequency response and amplitude response were examined. The time history responses of the cablestayed beam were also obtained by numerical simulations. It was shown that the cable length has a great effect on the fundamental frequency of the cable, and for a short cable, the flexural rigidity should be taken into account. The frequency response curve moves to the right, but the amplitude response curve moves to the left, when the flexural rigidity is considered. The amount of movement depends on the magnitude of flexural rigidity. The vibration of the beam has a significant impact on that of the cable, while the vibration of the cable has little effect on that of the beam.