A method is developed to simulate the fatigue crack growth in a carburized gear tooth considering the effect of residual stress. The calculation of stress intensity factor and the fatigue crack growth rates in the case layer are estimated by the experimental formula obtained in this study. The crack growth of carburized gears is measured by a crack gauge, and the validity of the simulation is confirmed. The critical length of initial crack which corresponds to the fatigue strength is introduced in the method, and a new design procedure of carburized gears is proposed based on the critical length of crack. As an example of the application of the procedure, the size effect is tried to discuss.