A generator potential transformer fault can induce a power plant trip. Voltage ratio and tan delta measurements are performed in KHNP for management of potential transformer aging. In this paper, partial discharge measurement for the generator potential transformer is introduced as an additional aging diagnosis method. IEC 60270 is adopted as the test criteria, and several MHz measurement center frequencies are also used for site environmental conditions including abundant signal noise. Developed partial discharge test equipment consists of an AC dielectric tester, a partial discharge pulse analyzer, and a partial discharge diagnosis system. Partial discharge pattern and inception voltage are analyzed based on epoxy insulators including the void and the metal particle.