Abstract: The quality of spot welds in 1 mm SUS304 austenitic stainless steel sheet metal was evaluated by using ultrasonic water immersion focusing method. Splash, solder to wear and other weld defects were distinguished through the A-scan signals and ultrasonic C-scan images. Results showed that the A-scan signals in defective regions was obviously different from the waveform in normal nugget regions, C-scan image can get effective information from the inner pattern of the spot weld based on ultrasonic water immersion focusing method. When the welding current exceeded 8 kA and the supply electrode was less than 2 700 N, it can get the information which included splash, solder to wear and other weld defects from the ultrasonic C-scan image. The measured diameter of spot weld ranged from 4.39 mm to 5.25 mm based on the ultrasonic C-scan image.