In a room with a raised floor HV AC system (RF system), the vertical temperature gradient became an important factor in relation to the ventilation requ1rement to maintain a vertical temperature difference within a comfort standard such as ASHRAE Standard 55-1992. A series of detailed laboratory experiments were carried out to obtain the design ventilation requirements with various conditions of ventilation rates, cooling loads, and types of floor outlets. The main results are shown as follows. (1) From measurements it is clear that the vertical temperature gradient is strongly dependent on the throw contributed to supply air volume, cooling load and type of floor outlet. (2) The relationship · of the vertical temperature difference in an occupied zone to cooling loads, and ventilation rates was obtained and the design ventilation requirements for the RF system were recommended.