Abstract: Pressurizer surge line contains high temperature, high pressure and high radioactive reactor coolant. It is of nuclear class I and seismic category I. It is one of the important equipments keeping the integrity of primary pressure boundary. Generally, the fatigue usage factor of surge line is comparatively high, owing to its operating temperature and pressure transients and thermal stratification cyclic loads. Consequently, SNERDI established a program to perform temperature measurement for surge line and fatigue re-evaluation of it accounting for thermal stratification effect for a 300 MWe PWR NPP. Refering to the temperature records of the measuring points relating to the plant heatup and cooldown, thermal stratification phenomenon can clearly be observed. And then the original design thermal transients were modified for the surge line, including the thermal stratification effects. The analysis models were established for the calculations of globle and local stresses due to stratification, and following ASME Ⅲ NB-3650 was re-evaluated. Its maximum design-basis fatigue usage factor during its design life is bellow 1.0. The consequent result not only is a representative of the scientific achievement, but also plays a significant role in the LTO & AM engineering projects.