Heat loss by window in building occupies about 1/4 energy amount of used building. And the parts of building which is has the most influence on thermal load are skins, outer walls and windows. The window is the worst weak point of building even if it has many advantage of solar radiation gain than other parts. Therefore, high thermal performance of window system can speak as very important part in save energy of building. In this research, it evaluates the thermal performance of window throught Therm6 & Window6 simulation program according to the location of the blind and angle of the slat after the venetian blind apply to the window system. And simulation results of Therm 6.2 & Window 6.2 were compared with measured values. Actual results, thermal performance accroding to the position and the slat angle of venetian blind increased approximately 21.6%.