A　precision　reducer　is　the　core　component　in　an　industrial　robot　and　its　performance　largely　determines　the　performance　of　the　whole　robot.　Lost　motion　is　the　key　index　to　characterize　the　performance　of　precision　reducers　and　its　measurement　and　evaluation　are　the　basis　for　improving　the　performance　of　precision　reducers.　However,　some　problems　in　the　lost　motion　measurement　still　remain　to　be　solved,　such　as　the　large　error　introduced　by　curve　fitting.　In　this　paper,　aiming　at　the　disadvantages　of　traditional　measurement　scheme,　the　measurement　scheme　of　lost　motion　is　optimized.　A　closed　hysteresis　curve　can　be　obtained　by　single　loading-unloading　cycle.　The　hysteresis　curve　model　of　a　precision　reducer　is　established　based　on　the　consideration　of　the　coupling　effect　of　nonlinear　stiffness　and　friction.　Based　on　the　established　hysteresis　curve　model,　the　evaluation　method　of　lost　motion　is　proposed　and　named　median　curve　method.　Compared　with　the　traditional　evaluation　method,　this　method　essentially　reflects　the　change　rule　of　hysteresis　curve　and　can　improve　the　measurement　accuracy.　Taking　the　RV　reducer　as　an　example,　the　measurement　experiment　was　carried　out.　The　measured　results　were　consistent　with　the　sample　index,　verifying　the　applicability　of　the　measurement　scheme　and　evaluation　method.　©　2025