Abstract: Being an important part of mechanical transmission, gears have been extensively applied in various machines. Gear produced by forging possesses the advantages of material reduction, low cost, higher efficiency, and better mechanical properties. In this study, the geometric model and finite element simulation technique are combined to analyze the thermal process of spur gears and the grain-refinement in microstructure during hot precision forging of spur gears is discussed. The results show that grain-refinement is obviously influenced by forging velocity, and the dynamic recrystallization distribution is in direct proportion to strain distribution, but in inverse proportion to static recrystallization distribution. The numerical modeling developed to calculate the carburized layer and to analyze the phase transformation can be used effectively to predict the change behavior of grains in spur gears during heat treatment.