a dynamic model for parallel-axis helical gears is proposed including friction between the contact teeth which can be used in dynamic analysis of variable speed process. In this model, the position of the action line and sliding velocity are determined by the angular displacement of the driving gear, and the translational displacements and angular displacements are chosen as generalized coordinates, so the model can be used in analysis of the process of variable speed of gear transmission system, furthermore the friction is calculated by variable friction coefficient. The acceleration process of an example is numerically simulated with the model proposed in this paper and the dynamics responses are compared with that from the model utilizing constant friction coefficient and without friction. Some conclusion can be obtained from the comparison: friction decreases the critical speed of meshing aparting of helical gear transmission and has effects of damping and excitation, which affects the dynamic responses of helical gears. Moreover, the variable friction coefficient has different effects on the dynamic responses of helical gears than the constant friction coefficient.