Recently the government strengthen the code for insulation and airtightness of buildings to reduce the energy for heating and cooling. U-Value of the windows have been strengthened 3.37W/m2K in 1987 to 1.5W/m2K in 2013. However, the reinforcement of the performance of building envelope is not always ensure the energy saving because of the building conditions such as internal heat gain, solar radiation. In this study, the relation between internal heat gain level and building envelope performance on heating and cooling energy demand was analyzed by simulation. The analyzed building is a curtain-walled office building in Seoul and the internal heat gain level and envelope properties were changed to analyze the impact on heating and cooling energy consumption. The results showed that the lowest energy consumption points are different according to U-Value and SHGC of window. Also, even though the lower U-Value of window was more used than higher U-Value window when the internal heat gain level is high.