Lubricant　film　thickness　is　one　of　the　most　important　parameters　to　indicate　the　operating　condition　of　machine　elements,　such　as　mechanical　seals　and　hydrostatic　slideway.　When　ultrasonic　waves　illuminate　the　interfaces　between　the　substrates　and　a　lubricant　film,　it　will　be　reflected　due　to　the　change　of　the　material　properties　at　the　interfaces.　These　reflected　ultrasonic　waves　contain　information　about　film　thickness.　In　this　paper,　wavelet　transform　modulus　maximum　method　was　explored　to　extract　the　film　thickness　from　its　reflection　ultrasonic　signals.　The　performance　of　different　wavelet　functions　within　various　scale　factors　was　experimentally　investigated,　and　the　optimal　wavelet　function　with　the　optimal　scale　factor　was　pointed　out.　It　has　been　shown　that　the　measurement　error　is　less　than　5%　when　the　thickness　of　liquid　layer　is　within　a　certain　range.　(c)　2012　Elsevier　Ltd.　All　rights　reserved.