It is essential to reduce building energy consumption in office building because government enact policy which encourages building energy certification from 2013. Office building has high cooling energy demand due to large glazed area of facade in these days. Shading devices can be an alternative of reducing high cooling energy demand. So, this study simulated a variety of exterior venetian blinds to know how much building energy be affected by orientation and reflectance of slat. The results of this study are based on Seoul weather data. The following is a summary of this study. 1) As a slat of venetian blinds has the lower reflectance, the more building energy reduced. Reflectance is usually affected by color and material of slat. In case reflectance is 0.2 reduce 4% of building energy than reflectance is 0.8. 2) Horizontal exterior venetian blinds are more effective than vertical exterior venetian blinds in all of orientation. Horizontal shape is average 16% more effective in shading effect than vertical shape. 3) In this case study, the most effective shading device is low reflectance horizontal exterior venetian blinds that result about 18% building energy reduction than no shade model. The results of this research can be used to plan shading devices for energy conservative office building.