A reflection-diffraction-deformation flaw detection method employs a transverse wave oblique probe. When an ultrasonic transverse wave encounters a defect during propagation, a reflected wave, a diffracted wave, and a deformed wave are generated. Through a comprehensive analysis of these waves, the presence or absence of the defect is determined by the reflected wave having reflection characteristics and the diffracted wave having the diffraction characteristics. The shape and size of the defect are determined by the deformed wave having deformation characteristics, namely the deformed surface wave generated at the endpoints of the defect which propagates on the defect surface. Furthermore, by the combination of paths trailed by the deformed surface wave, the deformed transverse wave, and the deformed longitudinal wave that are generated by the defect as well as that trailed by the transmit transverse wave, causes of all those waves in the screen can be revealed.