This paper discusses the essence of tooth modification of the toroidal worm according to the theory of curvature modification. It is pointed out that its essence is to make the reasonable shape along the teeth of the worm . So that the maximum ability of worm can be achieved. It also describes the essential differences between the toroidal worm’s modification and involute gear’s modification

and the difference in terms of the study methods and contents . Based on the new principle of curvature modification

the reasonable teeth shape — the curve of curvature radius is found out.