Abstract: Through the research for characteristic of stainless steel cladding and influence to ultrasonic examination due to the cladding, and theory analysis & experiment research for ultrasonic examination of crack under cladding, the ultrasonic examination technology under cladding is determined. TRL 60°/2 MHz and 70°/2 MHz are used for detection of crack under cladding, and TRL 45°/4 MHz is used for sizing of crack under cladding. This UT technology for detection and sizing for cracks under cladding for Nuclear Power Plant’s components is viable.