The results of ultrasonic testing performed during in-service inspection period for main components of nuclear power plant are affected by the performance of each inspector. To enhance the inspection reliability, the more restrictive qualification is required. KEPRI established Korean performance demonstration (KPD) system for the ultrasonic testing personnel, equipment and procedure which applied to the piping examination of nuclear power plant. A round robin test was conducted to evaluate the reliability of examination result by comparing conventional (DAC) UT and performance demonstration (PD) UT method. Participants took detection/length sizing test and depth sizing test using DAC method and PD method. The results showed that the PD method was preciser and more accurate than the conventional DAC method in length and depth sizing regardless of field experience and PD qualification status.