To effectively monitor the operation state of in-wheel motor used in electric vehicle and ensure the safety of the whole vehicle, a diagnosis method based on hidden Markov model (HMM) and Weibull mixture model (WMM) is proposed for mechanical fault of in-wheel motor, that is simply known as a WMM-HMM diagnosis method. Firstly, vibration signals of in-wheel motor are extracted for sensitive symptom parameters (SPs) which are used to characterize the operation state and establish the observation sequence. Secondly, Weibull mixture model is employed to expand the limited observation sequence under various operating states of in-wheel motor to obtain sufficient observation sequence as the training sample set of HMM, and HMM parameters are determined through combining supervised learning with unsupervised learning algorithm. Then the WMM-HMM diagnosis models are constructed under low and medium speed conditions respectively. Finally, the corresponding fault in-wheel motors are customized and the test bench is built to verify the proposed method. The test results show that the proposed method can accurately identify the mechanical fault state of in-wheel motor under different conditions and has good generalization and applicability in traditional methods comparison.