Abstract: To make sure that small diameter thin-walled pipes, which are in the aided process piping system of nuclear power plants, run well in in-service condition, we should find an effective way to inspect axial defects on inside and outside the wall of small diameter thin-walled pipe. This paper introduces the in-service UT inspection for small diameter thin-walled pipe. In the paper, we discuss the system composition, technical method, parameter selection for UT transducer and design principle of the UT inspection. Also, we study how to design and manufacture the local water jacket for the in-service inspection. And the main points of the inspection process are emphasized. By a great deal of testing and field inspection, it can be proved that the immersion focusing method is an effective way, which uses focusing ultrasonic probe and local water jacket, to inspect axial defects on inside and outside the wall of small diameter thin-wall pipe. And the test result is good.