Abstract: To deal with the non-conformance of dimension deviation that the inside/outside radius of the inlet and outlet not meeting the specification in the body castings of main steam isolation valve in some nuclear power plant, the corrective action of “transition welding between outside surfaces of MSIV and pipe” is proposed. As to this corrective action, analyses including the overall MSIV structure and function and stress analysis of the deviation area in the butt-weld ends are conducted. Then, to the special welding between the butt of the MSIV and the pipe, the bevel of valve is the same as the pipe side within the current thickness, while the slope of external surface after transition welding is less than 1:4. In addition, a complete set of NDT scheme is determined, including NDT for the groove of the valve butt and pipe before, during and after the transition welding. In the meantime, the production weld test coupon is made simultaneously, and the physical and chemical inspection should be carried out so as to verify whether the performance of the weld meets the requirements of technical specifications.