Aiming　at　the　need　of　the　safe　operation　of　mechanical　structure,　an　effective　method　to　measure　the　thickness　of　lubrication　film　using　ultrasonic　re　sonant　technique　is　developed.　A　model　of　three　layers　is　constructed　to　simulate　the　propagation　of　normal　incidence　ultrasound.　By　the　analysis　of　reflection　coefficient　of　lubrication　film　with　different　thicknesses,　the　resonant　frequency　of　reflection　coefficient　is　used　to　characterize　the　thickness　of　lubrication　film.　After　the　calibration　of　experimental　system,　the　experiment　of　lubrication　film　thickness　measurement　is　conducted　using　ultrasonic　resonant　method.　It　is　shown　that　the　measurement　results　agree　well　with　that　of　the　actual　condition.　Meanwhile,　the　influences　of　acoustic　properties　of　three　layers　on　the　reflection　coefficient　and　measurement　of　thickness　are　studied.　The　results　show　that　the　thickness　of　lubrication　film　can　be　characterized　by　the　resonant　frequency　of　reflection　coefficient.　The　error　of　measurement　remains　less　than　5%　on　the　basis　of　the　calibration　of　experimental　system.　It　has　little　or　no　effect　on　the　measurement　when　both　sides　of　lubrication　film　are　changed.　However,　the　measurement　system　needs　to　be　calibrated　before　changing　the　material　of　lubrication　film.