Abstract: The 3-dimensional FEM dynamic model of a cracked gear structure is established,the dynamic characteristics,i.e.,the natural frequency,the mode shape,the dynamic stress and so on,due to crack of gear tooth are analyzed in this paper.The influence of crack position and crack depth etc.on dynamic characteristics of gear is investigated by using computer simulation and the FEM.It shows that the influence of crack depth and crack position on both natural frequency and mode shape of gear is significant.The natural frequency drops and mode shape changes when a crack started.This natural frequency drops with the increase of crack depth,but the influence of crack position on both natural frequency and mode shape is greater than that of the crack depth.The natural frequency of gear drops more when crack locate at the tooth root in comparison with that of the gear when crack appears at the tooth tip.When a crack starts,the mode pattern of gear structure becomes very different from that of g gear uncracked.when a crack starts.The amplitude is augmented and the dynamic stress of gear changes in the crack neighborhood.The present study is important and usuful for damage detection and fault diagnosis.