The effects of CO concentration and reactant gas pressure on cell performance in phosphoric acid fuel cell have been studied by measuring current-voltage characteristics and by using ac impedance technique. The single cells were tested as a function of CO concentration of 0-0.986 vol% in fuel gas and of reactant gas pressure of 0-4 atm in the range of temperature of 160-220 degreesC. The presence of CO has significant effect on the electrode interfacial resistance above 0.721 vol% CO due to CO adsorption. These results were discussed as related to electrode reaction kinetics. In the pressurized cell. both internal resistance and electrode impedance of the cell decreased with increasing gas pressure, which are considered to increase the cell performance.