This study investigates the improvement of proton exchange membrane fuel cell (PEMFC) carbon monoxide by periodic air dosing. The carbon monoxide in the fuel gas leads to a significant loss in power density due to CO poisoning in the anode. The method involves bleeding air into the anode fuel stream , which contains CO in various concentrations (, , and ). In the transient CO poisoning test, air bleeding is performed for four different periodic air dosing and cell voltage is fixed at . The result of a dosing of air for in intervals of is similar to that of continuous air bleeding except for CO. The CO tolerance of the fuel cell and cell performance recovery from poisoning can be improved by air bleeding.