A　precision　reducer　is　the　core　component　in　an　industrial　robot　and　largely　determines　the　performance　of　a　robot.　Lost　motion　is　a　key　index　of　the　performance　of　precision　reducers　and　its　measurement　and　evaluation　are　the　basis　for　improving　the　performance　of　precision　reducers.　The　dynamic　measurement　of　the　lost　motion　is　affected　by　load　torque　and　speed,　but　there　is　no　uniform　standard　or　scientific　basis　for　measurement　speed,　thus　leading　to　the　deviations　in　measurement　results.　In　this　paper,　the　principle　of　the　dynamic　measurement　of　the　lost　motion　of　precision　reducers　was　firstly　introduced　and　the　method　of　separating　the　geometric　lost　motion　based　on　optimal　measurement　speed　was　put　forward.　Then,　according　to　the　Stribeck　friction　effect　of　precision　reducers,　the　method　of　determining　the　optimal　measurement　speed　was　proposed　and　the　calculation　model　of　the　optimal　measurement　speed　was　established.　Finally,　taking　the　RV　reducer　as　an　example,　the　measurement　experiment　was　carried　out.　The　friction　torque　of　the　RV　reducer　was　the　least　and　the　mean　value　of　the　measured　geometric　lost　motion　was　the　least　under　the　optimal　measurement　speed,　thus　verifying　the　proposed　method.