the calculation of contacted points´ principal curvature is the key to determine contacted area. Traditionally, the method of determining principal curvature is to make use of the theory of mesh and coordinate transform. However, this method is complex. In this paper, based on the feature of curve surface, moving frame and co-differential are employed to give a sententious, effective algorithm of principal curvature. Then, according to the formulation of contacted area, an application example is given. This algorithm can be used for calculation of principal curvature in all kinds of curve surfaces.