Mosques were created to fulfill the needs of Muslims in prayer, therefore thermal comfort is critical to consider so that the congregation is comfortable and can focus and feel at home while praying. The typology of ancient mosques reflects the community’s traditions and culture, as well as local knowledge values. Natural ventilation, which acts as a natural air exchange mechanism, is one facet of thermal comfort. The Gunong Kleng Mosque, founded in 1927, is one of West Aceh Regency’s ancient mosques. However, design adjustments have been made, such as the installation of wooden boards to on terraced roof. The air flow from the building’s roof is blocked, resulting in inefficient air exchange. The goal of this study is to determine thermal performance and the effect of air movement on thermal comfort at the Old Gunong Kleng Mosque. This study employed a quantitative approach using field measurements of air temperature, relative humidity, and air velocity. An anemometer and a USB data logger were utilized to collect data. Field measurements revealed that ET comfort in the mosque was 78% too hot, 16% warm comfortable, and 6% for optimal comfort. Ansys Fluet simulation with two types of situations was used to forecast air speed, demonstrating that changes in the design of the stepped roof had a significant impact on the air speed within the mosque. When opposed to an open roof, a closed roof slightly lowered the ambient temperature in the prayer room.