ABSTRACT Traditional displacement ventilation (TDV) and underfloor air distribution (UFAD) systems have been used widely because they create better indoor air quality. Many previous studies have compared the TDV or UFAD systems with mixing ventilation systems. This study first compared experimentally the TDV with UFAD systems that use four different diffusers (perforated TDV diffusers@ swirl diffusers@ linear diffusers@ and perforated-floor-panel diffusers) in an environmental chamber that can simulate different indoor spaces of the same size. The two systems had higher ventilation performance than the mixing one under cooling mode as well as under heating mode. Also@ the systems with low-height-throw diffusers (all except the linear diffusers) were better. This investigation used a validated CFD program to further study the ventilation performance of the TDV and UFAD systems for an office@ a classroom@ and a workshop of different sizes. The CFD results further confirmed the findings from the experiment@ but with more detailed information and at a lower cost. The air distribution effectiveness with the TDV system and with the lowheight- throw UFAD system was in proportion to the ceiling height of the indoor spaces.