Abstract: The overlay welding cracks were inspected by TOFD technique. Detecting features of TOFD of cracks were studied, and the influences of angle and distance of probe pairs on detecting results were analyzed. The results showed that TOFD technique could detect vertical cracks correctly located 3~10 mm under the surface. Combined with A-scanning signals, B-scanning and D-scanning images it could identify, locate and quantify crack-like defects effectively, and provide reliable and accurate information for the estimation of the quality of overlay welding.