Fuel test loop is an irradiation test facility which can conduct the irradiation tests of nuclear fuels and materials at HANARO. The FTL simulates the operating conditions of commercial nuclear power plants such as their pressure, temperature, flow and water chemistry to conduct the irradiation and thermo-hydraulic tests. The passivation of the fuel test loop was performed for the main cooling system in the commissioning stage with satisfaction of the operation criterion such as temperature and water chemistry conditions. The experimental results show that the passivation was completed successfully.