One　of　the　main　defects　that　influences　the　lifetime　of　a　gear　are　cracks.　The　phenomenon　induced　by　crack　initiation　and　propagation　is　the　continuous　degradation　of　meshing　stiffness,　which　will　determine　the　response　feature　of　the　entire　gear　transmission　system.　Based　on　the　assumption　that　the　gear　tooth　is　simplified　as　a　variable　cross-section　cantilever　beam,　a　time-varying　meshing　stiffness　calculation　model　is　established,　in　which　the　tooth　bending,　basis　deformation　and　contact　deformation　are　all　included.　The　meshing　stiffness　curve　of　the　healthy　gear　is　obtained　with　comparative　analysis　between　the　above　analytical　model　and　FEM　simulation.　Furthermore,　on　the　basis　of　a　tooth　root　crack　propagation　prediction　from　the　linear　elastic　fracture　mechanics　theory,　the　FEM　models　of　gear　tooth　with　cracks　of　different　lengths　are　found.　The　stiffness　calculation　within　one　and　a　half　meshing　periods　for　each　model　is　completed,　and　the　relationship　between　crack　size　and　stiffness　degradation　trend　is　established.　The　comparison　between　degradation　trends　in　different　meshing　regions　shows　that　the　degree　of　degradation　in　a　single　tooth　meshing　area　is　much　higher　than　that　in　a　double　teeth　meshing　region.　In　addition,　there　is　some　difference　between　the　double　teeth　meshing　areas.　©,　2014,　Nanjing　University　of　Aeronautics　an　Astronautics.　All　right　reserved.