Gear　tooth　crack　fault　signal　is　very　weak　when　the　crack　is　in　early　stage.　To　extract　early　gear　tooth　crack　fault　feature　effectively,　an　early　fault　diagnosis　method　is　put　forward　based　on　the　cyclostationary　demodulation.　In　this　method,　the　gear　meshing　harmonics　are　first　removed　from　the　vibration　signal　to　generate　the　residual　signal.　The　residual　signal　is　then　analyzed　by　using　the　cyclostationary　demodulation　method.　Simulation　results　and　actual　engineering　diagnosis　example　demonstrate　that　the　demodulation　analysis　method　proposed　in　this　paper　can　extract　the　early　crack　fault　feature　of　gear　from　complicated　vibration　signals　successfully.　It　is　more　favourable　to　detect　the　early　fault　and　judge　the　fault　degree.