Abstract: The content of critical chemical elements and the value of reference nil-ductility transition temperature (RTNDT) in the manufacturing process of reactor pressure vessel (RPV) forgings were analyzed. By introducing the modified hardenability coefficient into the hardenability coefficient function, the modified function which reflected the variation rule of RTNDT was established, and an example was also given to verify the effectiveness of the modified hardenability coefficient function. This study demonstrates that by using the modified hardenability coefficient function, the RTNDT value of forgings can be predicted relatively accurately.