For underfloor air distribution (UFAD) systems, more rapid mixing of the supply air with ambient airis desirable for better thermal comfort, and swirling air diffusers are usually used. In order to rigorously simulate the flow characteristics of such diffusers, we used the multi-grid technique and validated it with experimental results. In this paper, the technique is used to compare the square diffuser and the swirling diffuser. Based on the cooling load program ACCURACY, Computational Fluid Dynamics (CFD) technique is employed to investigate the impact of the different diffusers on thermal comfort quantified by the local percentage dissatisfaction due to draft (PD) in UFAD systems. From the simulation results, these two different diffusers show dramatically different airflow patterns near the floor, due to their different configurations. Both the swirling diffuser and the square diffuser are able to maintain room air stratification, which is a criterion for the successful design and operation of UFAD systems. Below the height of 0.1m, the largest temperature difference is about 5C, whichmeans that occupants will not feel much cold draft before the supplied cool air arrives at the ankle level (0.1m). Between these two diffusers, the most dramatic differences in vertical temperature gradients happen in the occupied zone. Due to the quicker combination with room air, for the swirling diffuser, the temperature difference between foot and head levels is smaller than that of the square diffuser. Above the height of heat sources (1.9m), the temperature gradients are much larger.