Abstract: The main coolant pipelines(MCL) is the large diameter and thick walled pipeline in the nuclear island of PWR. The MCL base metal is austenitic-ferrite stainless steel formed by casting, and its internal structure contains coarse columnar crystal and anisotropy. So it is rather difficult to apply ultrasonic inspection for the MCL welds. But with the application of multi-angle twin crystal focusing probes, multi channel automatic UT system, and by using test block of the same material with the pipe and layered scanning method, the inspection technology is well suited for the MCL welding. Present UT inspection is complementary with the RT for the weld and is well required to guarantee the quality of the welds.