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When a gear has a local fault, the vibration signals of gear may contain amplitude and phase modulation. Their spectrums contain meshing frequencies, harmonics, and coupling frequencies generated by modulation. According to the characteristics of vibration signals from the gears with faults, a feature extraction method of gear fault based on the envelope analysis and time-frequency image of S transformation was proposed. Firstly, vibration signals of wear gears of varying degrees were collected from the fault simulations. Then, envelopes were obtained by Hilbert transform of vibration signals and time-frequency contour maps of envelopes were achieved by S transformation. Finally, the features were extracted by calculating statistics base on the grey-scale matrixes (GLCMs) of the maps. The result shows that the proposed method can effectively extract gear fault feature.