The quality of spot welded joints was evaluated by the scanning acoustic microscope (SAM). The change characteristics of gray level of C-scan image were analyzed. The feasibility of evaluating the quality of spot welded joints and measuring the nugget diameter by using the change of the gray level was provided. And we analyzed the spot welded joints of 1.0 mm stainless steel under different welding parameters. The calculated values of nugget diameter are compared with the measured values by macrophotograph of samples. The results show that this method is available for measuring nugget diameter of the spot welded joints, and can avoid the influence of artificial factors and reduce production costs. The accuracy of testing result was verified by contrasting the metallographs of actual spot welded joints cross section.