The in-wheel-motor (IWM) drive system has some interesting features, such as the vibration of this structure at low velocity. An explanation of this phenomenon is given in this paper by considering the dynamics performance of the in-wheel motor drive system under small slip ratio conditions. Firstly, a frequency response function (FRF) is deduced for the drive system that is composed of a dynamic tire model and a simplified motor model. Furthermore, an equation between the resonance velocity with the parameters of the drive system is obtained by combining the resonance frequency of this drive system with the fundamental frequency of the motor. The correctness of the equation is demonstrated through simulations and experimental tests on different road surfaces. The impact of different parameters on the vibration can be explained by this equation, which can give the engineer some instructions to design a control method to avoid this feature.