Window opening behavior has significant influences on indoor environment and energy consumption in residential buildings. As a response to indoor environment, the control mechanism (window and interior door open/closed) should be studied first by comprehensive understanding of the variation of indoor environmental conditions. For this reason, a field measurement of environmental conditions was carried out in five representative apartments in Beijing from April to May in 2010. By comparison analyses of the field measurement results, major findings are as follows: (1) the concentration of room CO₂ can be the best predictor of occupant behavior, also window opening behavior, in residential buildings; (2) the variation in indoor air quality mainly results from large variation in window opening behavior; (3) apart from indoor and outdoor thermal environment, indoor air quality as well as occupants’ presence at room have also considerable effect on window opening behavior; (4) while defining window opening behavior for one room in residential building, it is necessary to take the window adjustment of its connecting room and the adjustment of the connecting door into consideration due to air diffusion between the two connecting rooms.