The present study is a combined experimental/analytical investigation of the effect of a cracked pinion tooth on the vibrational characteristics of a power transmission system. A system of nonlinear equations representing the analytical model of an experimental test rig is presented. The model includes the effect of crack size on the time-varying gear mesh stiffness. The relationship of the dynamic tooth loads to the crack size of a single cracked tooth of a spur gear pair is also shown. Comparison of the experimental and analytical vibration signals processed by using the wavelet transform show excellent agreement over a range of crack sizes and running speeds.