The curvature properties of ruled surfaces are derived. Several parameters are used to express the geometry of ruled surface explicitly. Using the ruled surfaces as generating surfaces, the equation of meshing is derived for a prescribed input-output relation. Considering the motion types of mechanisms and the particular ruled surfaces, the meshing equation is simplified. The pressure angle of the cam is analyzed as well as that of the follower, and the relation between these pressure angles is obtained using the equation of meshing. Two examples are given.