After an extensive research on the fundamental theory, the theoretical basis for the gearing with equal conjugate curvature is presented in this paper. This includes the theory for gear meshing with high oder of contact. In comparison to previous studies, the theory of this paper is more restricted going beyond third order parameters into the fourth order, the teeth profile had one order of contact higher than that of the ordinary gearing. So that further improved the contact strength of tooth profile. A large category of gearings with high order of contact was presented. The rack-pinion gearing with equal conjugate curvature is one of them. In my gearing the curvature center of media rank extend along the pitch line continuously and steadily.