Korea Atomic Energy Research Institute (KAERI) has conducted steady-state tests with the constant heat flux conditions to investigate the scale effect on heat removal behavior of the air-cooled Reactor Cavity Cooling System. The heated cavity heights of KAERI is 4.0 m, a quarter-scale of the full scale cavity height. The post-test analysis results using GAMMA+ was performed to simulate the test results at the above test conditions. GAMMA+ showed the good predictability for the air-cooled RCCS test results . The governing mechanisms are the radiation across the cavity and the convection in the riser duct, based on the comparison with test and analysis results. Especially, the mixed convection in the riser is very important to extrapolate RCCS coolability from the test results.