Abstract: Due to the alternation between single and double pairs of teeth meshing, continuous change of meshing point along the line of contact and the elastic deformation of teeth, the load of gear teeth is quite complicated during the meshing. It is difficult to calculate the load of teeth accurately during the meshing. In this paper, according to the analysis of the meshing process of involute spur gear, a mechanical model is developed to study the load distribution among the teeth. Then the deformation compatibility in the meshing process is analyzed, and the calculating models of various deflection and elastic contact deformation of teeth are derived. A method for calculating the load and stress of involute spur gear is developed. Taking a pair of involute spur gear for example, the load of gear teeth, teeth root stress and elastic contact stress are calculated, and the curves described the variation of the load and stress are obtained respectively. Using the calculating method proposed in this paper, the more precise load and stress of teeth at any position in meshing cycle can be calculated. It is a new effective way for accurately analyzing the mechanics of involute spur gear.