A numerical approach using the finite element method was shown to analyze the load distribution of toothed belt drives at unsteady states. When the driven pulley was subjected to fluctuating torque due to, for instance, cam driving, the tension side of the belt periodically becomes the slack side according to torque fluctuation. The reserve also occurs. Hence, the tooth load distribution changes with respect report to time and it is quite different from the tooth load distribution under constant torque loading since the frictional force changes its direction.