PCM (phase change materials) is an innovative technology and an effective method for improving the thermal mass of buildings owing to it possesses the property of large thermal capacity within a limited temperature range, which is similar to an isothermal energy tank. However, most of the previous studies focused on the combination of PCM and walls that the utilization potential of rooftop is lack of concern though there is a significant amount of heat gains from rooftop. Several previous studies used experimental or numerical simulation methods to investigate the energy-saving potential of installing PCMs in roofs and analyzed the relationship between energy conservation, climatic conditions, and the features of PCM such as thickness, positions and PCT ~. The utilization of PCM is now gradually used for better building heat insulation. In addition, for building located in different climate zones during different seasons, it is important to select appropriate PCMs possessing suitable phase-change parameters to meet the energy-efficiency requirements of a building. Specifically, a PCM with ideal performance in the cooling season may yield poor or unfavorable results in the natural-ventilation season. There are few previous studies attempted to analyze the efficacy of PCMs in reducing the cooling load during the air-conditioning (AC) season and the thermal discomfort during the natural-ventilation (NV) season simultaneously. Consequently, this study aims at investigating the effectiveness of roofs with PCMs under the thermal management conditions in terms of reducing the cooling load demand during the AC season and ensuring thermal comfort of classrooms during the NV season. It is expected to suggest the appropriate properties of PCM for school buildings, including thickness and PCT. In addition, the optimal pattern of PCM which is proper to be utilized through the entire year is proposed to fulfill the indoor comfort and energy-saving potential.