Abstract: 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.