Radiation workers who handle radioisotopes, radioactive waste, nuclear material etc. may be contaminated with radioactive material due to inhalation, resulting in internal radiation exposure. For preventing radiation damage and monitoring the exposure of workers, KAERI operates a Body Radiation Measurement Laboratory. According to Article 5 of the Nuclear Safety and Security Commission (NSSC) Notice No. 2017-77, “Regulation on Measurement and Calculation of Internal Radiation Dose,” The nuclear energy-related business operator with workers etc. shall establish and operate procedures and methods including the following Subparagraphs to secure the reliability of measurement of the internal radiation dose : operation and calibration of measuring instrument, inspection procedures, uncertainty of measurement, lower limit of detection and geometric configuration used for measurement. In accordance with the provision, Whole Body Counter utilized in the Body radiation Measurement Laboratory has periodic calibration / QA procedures to ensure reliability. This paper performed reliability validation of the measurement system of the Body Radiation Measurement Laboratory in the KAERI based on the performance criteria for radio-bioassay criteria presented in ISO 28218 and ANSI HPS N13.30-2011(R2017). The first criteria is MTL (Minimum Testing Level). ISO 28218 provides MTLs for each measurement category, type and nuclide. For reliable results, it is recommended to use calibration sources with higher radioactivity than the values given. The MTL for fission products in total body counting is 3 kBq and for the last 3 years the laboratory has been using sources of 6-7 kBq (Co-60, Cs-137 etc.). The second criteria is RMSE (Root Mean Square Error). It is a measure of total error defined as the square root of the sum of the square of the relative precision (SB) and the square of the relative bias (Br). The RMSE shall be lower than or equal to 0.25. The largest RMSE in the last 3 years is 0.12, and average value is 0.065, which meets the criteria. In this study, we verified the reliability of the radioactivity measurement system (WBC) based on the radio-bioassay standards presented in ISO 28218 and ANSI HPS N13.30-2011(R2017). The values were obtained using 3 years of calibration count data, and it was found that both MTL, RMSE for each nuclide met the standards with a large margin of error and were in good operating condition. This study can be applied to the maintenance, performance check, and reliability verification of similar in vivo radio-bioassay methods.