Abstract: Aiming at the characterization of the depth and orientation of a surface crack, a nondestructive testing method using critically refracted longitudinal (L_CR) wave was proposed. The carbon steel test block containing vertical cracks (1~8 mm depth) and oblique cracks (10 mm length, 15°~90° inclination angle) were tested. The effects of depth and orientation were analyzed. The amplitude of L_CR wave and the B-scan imaging were used to determine the depth and orientation. Results showed that L_CR wave was blocked and diffracted wave from the crack tips was also observed. As the crack depth increased, the blocking effect of L_CR waves increased, leading to a decrease in the amplitude of transmitted L_CR waves. However, it was not sensitive to the crack orientation. At the same time, the arrival time of L_CR wave was delayed. B-scan images were reconstructed by the change of the diffracted wave with both crack depth and orientation, which provided a new idea for the qualitative assessment of surface cracks.