It is one of major objectives to achieve a uniform thermal environment and to minimize the local discomfort, when designing or operating air distribution systems. An active chilled beam (ACB) system, which is gaining much popularity owing to reduced fan energy consumption, needs to be evaluated in terms of uniform thermal environment, because it is operated with much smaller air flow rate than conventional all-air systems. The uniform thermal environment, or thermal uniformity, can be evaluated by analyzing air diffusion performance index (ADPI), local discomfort parameters within conditioned space. This study presents an evaluation method on the thermal uniformity in a room with different air distribution systems. To do this, a test bed facility was constructed so that four different air distribution systems, including ACB system, can be separatively operated. The ADPI of the ACB system was 80.7%, which is lower than that of all-air system, however, it could satisfy the recommended level (80%) of ADPI by ASHRAE. Although the ACB system resulted in the relatively low draught risk, averagely 6.7%, it was found that some regions near the chilled beam can be exposed to the draught risk due to the descending cold air. Vertical air temperature difference was not higher than $1.0^{circ}C$, showing that the ACB system resulted in efficient air mixture in the vertical direction. It can be concluded that the ACB system can attain an acceptable thermal uniformity with much less air flow rate than other conventional air distribution systems.