In school, an improvement on thermal environment which can develop students\' abilities and can enhance teacher\'s efficiency is an important factor. Recently, some schools installed an air conditioner in classroom, while most schools selected the built-in type air conditioner. However, the study on a valuation of thermal environment is not sufficient, especially in classroom. It is insufficient that the study is a valuation of thermal environment which considered valuation about indoor thermal environment diffusion angle and air volume. In this work, the change of thermal environment was investigated for five kinds of air diffusion angle(15° , 30° , 45° , 60° , 90° ) for ceiling type air conditioner. The vector and temperature distributions of air in the room were calculated by a 3-dimensional numerical method. This study was also conducted to calculate the Mean Age of Air and Predicted Mean Vote(PMV). This analysis showed that 30-45° is the optimum angle of diffusion in cooling mode and in heating mode. Keywords：Thermal Comfort, Diffusion Angle, Mean Age of Air, PMV, Average Ventilation Efficiency