Abstract: The area of defects in brazing weld needs to be analyzed while evaluating the result of ultrasonic inspection. The current criteria standards normally specify the maximum allowable defects and define the allowable ratio of the area of defects to that of entire weld. Now it is feasible to inspect brazing weld by C-scan technology of ultrasonic inspection. However, there is no satisfied method to calculate the area of defects. Based on the analysis of the feature of C-scan image, this article mainly proposes the detailed solutions and compares several different methods to select threshold value, segment C-scan image and calculate the area of defects.