Scale model measurements have been conducted to identify the differences of daylighting performance under real sky and in sky simulator developed by LAEL of KHU. Under overcast sky condition, two kinds of scale model experiments were conducted by using Agilent data logger and photometric sensor Li-cor. Firstly, a 1/20 scale model of an office room with 4.9m width, 7.2m length, and 2.6m height was made. Five measurement points were set at 1.2m, 2.4m, 3.6m, 4.8m, 6.0m from the window. During measurement, the luminance of zenith and horizon in a sky simulator was 2,550cd/m² and 851 cd/m². The luminance of zenith and horizon in real overcast sky was 11,479 cd/m² and 3,283cd/m². The mean of the measurements between sky simulator and real sky was 7.1% of difference. Secondly, a 1/30 scale model of toplit space with 15m width, 15m length, 15m height was made. The measurement point was set at center of the atrium floor and the well index of the model were set in 0.25, 0.5, 0.75, 1.0, 1.25. During measurements, the luminance of zenith and horizon in a sky simulator was 2,550cd/m² and 851cd/m². The luminance of zenith and horizon in real sky was 6,737cd/m² and 2,643cd/m². The mean of the measurements between sky simulator and real sky was 1.7% of difference. It is proved that the KH University sky simulator is fully validated for accuracy and usability for daylighting researches.