This paper describes a series of experiments, which investigated the performances of the underfloor ventilation systems in thermal comfort and indoor air quality aspects. Both the floor return (FR) and the top return (TR) types were tested. An additional set of experiments were conducted on the traditional ceiling-based system for comparison. Results reveal that the underfloor systems created significant overcooling in the nearby area of the floor supply terminals and temperature stratifications in the room. The underfloor systems achieved some level of improvements in ventilation performance. The FR system exhibited a more localized ventilation characteristic than that of the TR system. With the support from the experimental results, some design considerations were suggested for the underfloor ventilation systems.