The variable cross section cycloid drive can realize lower backlash by adjusting the axial location of the variable cross cycloid gear pair.The curvature radius is an important parameter in design and machining process such as undercut

contact stress calculation and determination of machining tool radius.The profile surface’s curvature of variable cross section cycloid gear is studied based on kinematics.The paper covers the following topics:（i） the curvature of profile surfaces of variable cross section cycloid gear is studied

and general equation of variable cross section cycloid gear’s curvature is derived based on the curvature of pins and the motion parameters;（ii） the profile surface’s curvature of linear variable cross section cycloid gear as conic cycloid gear and nonlinear as drum cycloid gear are developed

and related numerical calculation examples are given.