This study aims to find an appropriate window for apartment to provide proper sound insulation. To achieve this goal, measured windows with various thicknesses of the sound insulation performance of inner air space and sound absorption materials in the inner air space. As a result of this study the improvement of the sound insulation performance of windows was shown through the analysis and the measuring of window these characteristics. These results may be applied to the manufacture of window frames and provide basic data for the improvement of the sound insulation performance of windows.