PROX is known as one of the most promising technologies which prevent the anode of PEMFC from being poisoned by carbon monoxide. Hence, commercialization of the PEMFC system is highly dependent on the development of the corresponding PROX system. This study is focused on the development of the PROX system for 1 kWe PEMFC, and the results can be used to predict the performance of a higher-scaled system. Pt-Ru/Al2O3 catalyst made by incipient wetness method has been used for the reaction, since this catalyst shows high activity and selectivity for CO oxidation over a wide range of temperature. With the catalyst, a 1 kWe proto-type PROX system was set up and its performance was evaluated for the steady state as well as the transient conditions. The outlet CO concentration of the system was below 10 ppm at its steady state. Also, even at transient conditions, in which sudden flow rate change occurred, the resulting CO concentration still remained under 10 ppm.