Analytical results are presented for nonlinear vibrations of a wide timing belt with parametric excitation in tension and meshing impact forces. The equation of motion is developed from Hamilton's principle, and approximate solutions to the basic equation are obtained using the Galerkin method. The steady-state responses are calculated using the harmonic balance method. Analytical results are presented which show the contributions of pulley speed and tension to natural frequencies. The resonant amplitude varies with cross section y. In the case of variable tension, the resonant peak value varies with the rates of variable tension, and tension. Amplitude reduction by means of periodic variation of tension is effective for tension and rate of variable tension.