High　order　resonance　of　the　ultrasonic　transducer　was　investigated　and　adopted　to　inspect　spot　weld.　The　transducer　can　receive　the　half-wavelength　resonance　frequency　signals　from　non-welded　thin　steel　sheet　because　of　the　ultrasonic　multiple　reflect　in　steel　sheet,　and　the　frequency　of　signals　is　near　the　high　order　resonance　domain　which　is　out　of　the　bandwidth　of　the　transducer.　These　signals　can　be　picked　up　through　wavelet　packet　transform　(WPT)　because　of　their　special　spectrum　localization.　Thus,　the　WPT　can　be　adopted　to　evaluate　the　joining　quality　of　resistance　spot　welding　of　zinc-coated　steel　sheet.　Through　analyzing　the　signals　in　different　scale,　the　characteristic　number　which　can　determine　the　quality　of　the　spot　weld　was　obtained.　This　non-destructive　evaluation　may　be　used　to　quantitatively　estimate　the　quality　of　spot　welds.　As　the　low　frequency　transducer　is　used,　the　testing　cost　is　obviously　decreased　in　comparison　to　the　normal　testing.