In this paper, a model of a gear with a crack for dynamic analysis is established based on finite element method. Internal dynamic excitation in the process of transmission is calculated by numerical simulation method. Dynamic response analysis with a crack is carried out and simulated by using a full method successfully. Fault signals are analyzed in time and frequency domain, and compared with signals from a normal gear. Results show that cracks have obvious dynamic effects on gear structure, which lay the foundation for the fault diagnosis and analysis.