Abstract: In order to improve the work efficiency of non-destructive testing (NDT) and the reliability of NDT results, an automatic method to detect defects in ultrasonic image was researched. According to the characterization of ultrasonic D-scan image, clutter wave suppression and de-noising were presented firstly. Then, the image is processedby binaryzation using KSW 2D entropy based image segmentation method. The results show that, the global threshold based segmentation method is somewhat ineffective for D-scan image because of under-segmentation. Especially when the image is bigger in size, small targets which are composed by a small amount of pixels are often undetected. Whereas, local threshold based image segmentation method is effective in recognizing small defects because it takes local image character into account.