The purpose of this study is to improve energy efficiency of windows in office buildings through the evaluation of their heating, cooling and illumination load. Energy efficiency is influenced by window size which is determined at the early stage of building design. The process of this study is as follows. First, energy performance is analysed according to the various rates of windows through computer simulation (ECOTECT). Then, the annual heating, cooling and illuminating loads according to the different window sizes are compared one another. Results indicated that the optimal window size considering energy efficiency is 50% of the surface area. When the window size is 50% of the surface area, annual maintenance expense is also smallest. Since the cost of cooling is larger than that of heating, too low indoor air temperature in summer is unfavorable based on the reasonable annual maintenance expenses.