[en] Radioactive noble gases (³⁷Ar, ³⁹Ar, ⁸⁵Kr, ¹²⁷Xe, ^131mXe, and ¹³³Xe) and tritium are emitted from nuclear reactor when the electricity is produced by nuclear power plant. For standardization of the radioactive gases, length-compensated proportional counter (LCPC) has been developed as the primary system at National Physics Laboratory (NPL), National Metrology Institute of Japan (NMIJ), National Institute of Standards and Technology (NIST), Pacific Northwest National Laboratory (PNNL) and other metrology institutes. We are developing a new primary system of radioactive gas standardization by the LCPC at Korea Research Institute of Standards and Science (KRISS). KRISS’s LCPC employs three stainless tubes of 25 mm inner diameter with different lengths (150 mm, 200 mm and 350 mm) placed in a big airtight chamber with 25 μm diameter of the gold coated tungsten wire located in the center of each tube. For the energy calibration, ⁵⁵Fe X-ray sources are installed at the end of each counter. The corrected counting rate of ⁸⁵Kr was calculated from the counting rate, background counting rate, threshold correction factor derived from Monte-Carlo simulation, and dead time. Finally, the corrected counting rates per unit length are 2.41 ± 0.10 s^-1·mm^-1 for 2.12 ± 0.08 mL and 4.29 ± 0.44 s^-1·mm^-1 for 3.64 ± 1.05 mL. (author)