Lost　motion　is　the　key　index　for　describing　the　performance　of　precision　reducers.　Although　its　definition　appears　simple,　it　involves　complex　measurement　and　evaluation　problems.　In　this　paper,　the　principle　and　method　of　precision　reducer　lost　motion　measurement　were　discussed　from　the　aspects　of　static　and　dynamic　measurements.　The　similarities　and　differences　of　different　measurement　methods,　and　their　respective　advantages　and　disadvantages　were　analyzed.　In　addition,　the　commonly　used　evaluation　methods　in　the　industrial　field　were　studied,　and　it　is　emphasized　that　the　differences　in　the　evaluation　methods　are　primarily　responsible　for　the　differences　in　the　obtained　results.　Finally,　it　is　noted　that　the　establishment　of　a　dynamic　measurement　system　for　the　comprehensive　performance　of　precision　reducers　is　imperative　for　meeting　the　industrial　demand.　At　the　same　time,　it　is　also　important　to　establish　a　complete　set　of　specifications　and　evaluation　criteria　for　precision　reducer　lost　motion　measurement.　©　2018,　Science　Press.　All　right　reserved.