Abstract: To compensate the coupling between probes and curved surfaces in ultrasonic testing of small diameter pipes, the contacts between the ultrasonic plane probe and curved surface probe and the curved surfaces of small diameter pipe are analyzed, then the relation between curvature correction factor and normalized radius of curvature is based to derive the gain to be compensated. In a test to correct the coupling curved surface, the plane of selected probe and the grinded curved surface are applied to scan the standard reflector of SGB test block, and decibels of equivalent reflection are recorded under 80% of the wave compete. Results show that the greater the arc radius difference between curved surface probe and SGB test block is, the smaller the echo of the standard reflector is, the larger the correction factor of coupling curved surface needs. The coupling effect is greater when curvature radius of probe curved surface is larger than that of test block.