Recently the Government strengthen the Standards of walls and windows thermal performance to reduce the energy consumption. U-Value of domestic windows Standards strengthened 3.37W/m2ㆍK in 1987 to 1.5W/m²ㆍK in 2013. However, These reinforcement of the building envelope is come from misunderstanding about insulation, and it is not always ensure the energy saving in office buildings. In this study, the effect of the performance(U-value) of building envelope on heating and cooling load according to the level of internal heat gain in office building was analyzed by simulation method(EnergyPlus). The results showed that the cooling load is no longer decreased at a certain level of window U-value. The cooling load is increased when the U-value is under a certain level.