The proper axial modification can prevent the load concentration,improve the meshing performance,reduce the noise and improve the carrying capacity. Based on the tooth load distribution model,the meshing longitudinal error and gear tooth deformation,the calculation methods of the effective contact width and crowning value is established,and the axial crowning curve equation is given. Gear pair models with different crowning values are established,and the longitudinal load distribution is calculated by 3 D contact finite element method. According to the theoretical crowning value determined by this method,the minimize value of longitudinal load distribution factor can be achieved.