The purpose of this research is to investigate the occupants' thermal comfort of buildings served by under floor air distribution (UFAD) systems.A questionnaire was distributed to building's occupants to measure their thermal comfort satisfaction. In addition, a physical experiment was donein one of the typical rooms where an occupant performs typical office work during typical hours which equipped with thermal comfort device. Byboth tools of investigation, occupants feel between slightly cool and cool; 80% of survived occupants have this feeling and average predicted meanvote (PMV) was -0.7 (slightly cool) of the subjective body. This was based on supply air diffuser temperature of 18°C and a supply air velocity of1.5 m/s. Then, a Computational fluid dynamics (CFD) model of the room was constructed and validated compare to the actual collected datausing building simulation software. This model was used to perform a parametric study by changing the operating conditions; this consolidatedeleven sets of numerical simulations. The Air Distribution Performance Index (ADPI) was used to measure the relative level of thermal comfortin the tested configurations. The new proposed of operations conditions improved the ADPI by more than 90%, where the optimum occurred at21°C supply air temperature and 1 m/s air velocity.