Based on the torsion deformation the gear body and the contact deformation of tooth surface, the fundamental equations of load of the gear was presented, thus established the theoretical basis of the load derivation of the gearing. In this paper, the torsion deformation of the circular arc gears is analyzed first. Then the second order differential equation for the tooth surface load distribution of the involutes gears has been deduced. The equation for the load derivation of the spur gears is presented while the errors of original tooth surface are under consideration. Revision of tooth surface in order to compensate the load deviation of the plastic gears is presented in this paper.