Applying the theories of Hertz contact, tribology, gear engagement and heat transfer, the planetary gear was taken as study object, the variation of sliding velocity and friction coefficient was investigated and the contact stress along the contact path was conducted. Heat transfer coefficient of gear end face and teeth surface and heat flux at different engaging positions were calculated. 3-D finite element models of gear teeth were established to predict stead temperature field. The influence of standard and modified profiles on temperature field and contact pressure was evaluated. Based on the results of stead thermal analysis, distribution of transient temperature field was investigated by using finite element method. 