The baseline and verification performance tests based on American Society of Mechanical Engineers (ASME) Performance Test Code 6 (PTC 6) for Maanshan Nuclear Power Station (MNPS) Unit 1 were successfully conducted prior to and after the replacement of a high-pressure turbine rotor. In order to verify that the actual improvement in electrical output resulting from this replacement meets the vendor’s guarantee, measurement uncertainty analysis of the thermal performance test was calculated. Two verification performance tests show that the deviation for the corrected heat rates of the two tests differs by only 0.11%, which therefore meets the ASME PTC 6 requirements where the deviation between two tests should be within 0.25%. Thus, the quality of the test results is acceptable. After accounting for the test uncertainty, the test results demonstrated that the improvement in gross electrical output compared to the baseline performance test is between 12.57 and 22.63 MW(electric), which is higher than the contract guarantee of 10.0 MW(electric). Moreover, the major parameters of the turbine cycle measured during verification performance tests have established a reference base for monitoring the plant operating performance and provide useful information to turbine cycle design for a nuclear power plant.