A window system is an essential component for thermal and visual comfort in built environments. Although required performances of a window are various and complex, only simple aspect(s) of the system is considered in the process of the selection of a window system in practice. Mostly, single properties such as U-value and SC are mainly used to represent the performance of a window. But such single properties are not capable to express the performance of the window systems in an occupied space based on the performance concept. The aim of this study is to draw the performance requirements and to develop performance indicators for window systems installed in a living room. First, this study investigates a literature review to compose aspect systems of a window. A questionnaire survey is also conducted to confirm the required performance aspects and performance requirements. The aspect system and performance requirements are used to develop the WPI (Window Performance Indicator). The developed WPI can facilitate optimal selection of window systems in apartment houses in the design and construction stage of a building.