It is important to know how to balance the energy saving with the improvement of a living environment in guest rooms of a hotel. This paper shows the influence of window-glass specification and overhang of window onto the air-conditioning loads (hereinafter referred to as AC loads) of the guest rooms of the hotel by using the dynamic simulation software “THERB” (the simulation software of the thermal environment of residential buildings) which can estimate temperature, humidity, sensible temperature, and heating/cooling load for multiple zones of buildings. Discuss and seek out the specification of window with lowest AC loads with simulation comparing is considered as a method for sustainable window designing. As a result of numerical calculations applying the simulation, main effects to select the window glass and apprentice of window are shown as follows. (1) The influence of the window area is the largest in summers because of the influencing of the solar heat gain coefficient. Smaller window and apprentice of window is decreasing AC load in summers and increasing AC load in winters. (2) Double-glazing is decreasing the AC load both in summers and winters. The annual load becomes smaller by about 4.4% with using double-glazing window.