In this study, classrooms environment was measured in the open-plan elementary school introduced natural ventilation windows. The actual effects of natural ventilation windows at classrooms are figured out from the field survey of thermal environment, indoor air quality and usage of classrooms. The result of the study, the rate of windows open could be related to weather, wind velocity and outdoor air temperature. Furthermore, it was confirmed that natural ventilation windows control air volume from pressure difference, and enough amount of ventilation was available. In terms of thermal environment indoor temperature was much higher than outside temperature when windows were closed for all night. Night purge was effective in lowering indoor temperature in the later research.