A computational model of the fatigue crack initiation period on the helical gears ,with consideration of the heat treatment by carburization and friction effects, is presented. In order to calculate the number of stress cycles required for the appearance of the initial cracks, and study the influence of dynamic behavior. For this end, a dynamic model of gear with two freedom degrees in torsion developed in Matlab and fatigue criteria are used. The computational results are compared with those obtained using strain- life method. The obtained results show that appearance of initial fatigue crack depends strongly on the friction coefficient between the surfaces, heat treatment, and on the dynamic load.