Abstract: The incidence point of probe changed due to the variation of the distance between two probes while TOFD(time of flight diffraction) method was used. It was concluded by sufficient experiments that the refraction angle of probe enlarged while the distance between two probes increased. Meanwhile, the front distance and delay time of probe changed in an extent which could be neglected. Different flaws were tested by TOFD method to get the refraction angle of probe for the highest diffraction efficiency of ultrasonic waves from the tip of flaw. It was shown by tests that the actual refraction angle of probe was approximately 70°to 80°, and it was not affected by the labeled refraction angle.