On the basis of heat transfer theory,the bulk temperature field of constantly no clearance precision steel ball transmission is studied and the formulas of relative sliding velocity,maximum contact stress,and friction coefficient are derived.Meantime,the mathematical models of heat flux of engagement pairs and convention heat transfer coefficient of each region are established. By the use of FEM software,the bulk temperature field of engagement pairs are evaluated. Besides,the effects on bulk temperature field caused by design parameters and working parameters are analyzed. It is shown that the bulk temperature of central plate is higher than that of planetary plate and the bulk temperature field is apparently influenced by design. The research findings lay a theoretical foundation for the thermo-elastic coupling analysis and reliability study of constantly no clearance precision steel ball transmission.