Estimating　the　torsional　stiffness　has　always　been　the　primary　issue　in　analyzing　the　dynamic　characteristics　of　cycloid　gears.　The　traditional　method　of　obtaining　torsional　rigidity　involves　calculating　the　ratio　of　the　input　torque　and　rotation　angle,　treating　the　deformation　of　cycloid　gear　as　a　black　box.　In　order　to　thoroughly　understand　the　rotation　angle　caused　by　the　local　contact　deformation　of　each　cycloid　pin　gear,　a　Majumdar-Bhushan　contact　model　and　the　finite　element　method　are　combined　to　express　the　normal　contact　stiffness.　By　multiplying　the　normal　contact　stiffness　of　each　pin　gear　and　the　arm　of　normal　contact　force,　the　torsional　stiffness　of　the　cycloidal　pin　wheel　system　can　be　calculated.　Experiments　are　conducted　to　establish　the　relationship　between　the　torsional　stiffness　and　roughness　parameters　of　the　machined　tooth　surface.　The　effect　of　input　torque　on　the　torsional　stiffness　has　also　been　analyzed.　This　study　formulates　a　relationship　between　the　torsional　stiffness　and　surface　characteristics　of　cycloid　gears,　which　can　help　improve　their　design　and　manufacture　in　the　future.