The　lost　motion　is　the　key　index　to　represent　the　hysteresis　feature　of　precision　reducer.　Its　measurement,　evaluation　and　control　are　important　ways　to　improve　the　performance　of　the　precision　reducer.　However,　the　measurement　of　precision　reducer　has　some　limitations,　e.g.,　low　efficiency,　large　error　of　measurement　data,　imperfect　evaluation　system,　etc.　In　this　paper,　the　measurement　and　evaluation　system　of　precision　reducer　lost　motion　is　studied.　A　new　method　of　obtaining　hysteresis　curve　and　geometric　lost　motion　is　proposed,　which　avoids　the　influence　of　error　introduced　by　curve　fitting　and　improves　the　measurement　precision.　The　evaluation　system　of　the　whole　life　cycle　is　constructed,　including　evaluation　index,　evaluation　model　and　evaluation　method.　The　comprehensive　evaluation　of　the　precision　reducer　lost　motion　is　realized.　Based　on　the　RV　reducer　comprehensive　performance　tester　developed　by　the　research　group,　taking　RV　reducer　as　an　example,　the　actual　measurement　experiment　is　carried　out.　The　maximum　value,　minimum　value,　mean　value,　and　standard　deviation　of　geometric　lost　motion　are　obtained.　They　are　1.4275　arcmin,　0.38　arcmin,　0.8394　arcmin　and　0.2588,　respectively.　The　result　of　measurement　is　consistent　with　the　sample　index,　which　verifies　the　application　effect　of　the　measurement　scheme　and　evaluation　system.　The　measurement　scheme　and　evaluation　system　proposed　in　this　paper　is　one　kind　of　universal　method,　which　is　suitable　for　the　measurement　and　evaluation　of　lost　motion　of　various　precision　reducers.　©　2018,　Science　Press.　All　right　reserved.