Passive displacement cooling (PDC) system is an emerging cooling technology in recent years to provide thermal comfort via buoyancy-driven natural convection. While PDC technology has higher energy efficiency compared to conventional mixing ventilation (MV) systems, few studies investigated the thermal comfort of PDC systems in an actual office room. Here, this study evaluated the difference between PDC and MV in terms of indoor airflow distribution and thermal comfort for a 400 m² office room in Singapore. A computational fluid dynamics (CFD) model was developed to simulate the thermal environment and thermal comfort was analyzed via four indices: the supply cold air velocity, the indoor mean air temperature, the predicted percentage of people dissatisfied due to draft (DR), vertical air temperature difference between head and ankles. It is found that PDC system had less draft sensation and acceptable thermal stratification compared to MV systems. The average DR in the occupied zone of the PDC system (5.3%) is better than that of MV system (7.6%) for the 44 occupants.