Abstract: Water immersion ultrasonic resonance method was used, together with the all-phase spectrum analysis (APSA) technique, to detect the thickness of the thin layer of alumina ceramics on the 304 stainless steel substrates in the thickness less than 1 mm. The results show that the relative error of coating thickness by APSA-based ultrasonic resonance technique method is less than 2%, which means this method can measure the thickness of thin alumina layer more accurately.