Proton exchange membrane (PEM) fuel cell is regarded as one of the potential renewable energy which may provide a possible long-term solution to reduce carbon dioxide emissions, reduce fossil fuel dependency and increase energy efficiency. Even though great progress has been made, long-term stability and durability is still an issue. The contamination ion plays an important role on the electrical performance of PEM fuel cell. This paper investigates the effect of Mg2+ contamination on PEM fuel cell performance as a function of Mg2+ concentration. Two levels of Mg2+ concentration was chose. From the experimental results, it can be obtained that a significant drop in fuel cell performance occurred when Mg2+ was injected into the anode fuel stream. The voltage and power density of fuel cell decreased larger and larger with increase of Mg2+ concentration over time. The Mg2+ mainly caused the concentration polarization loss from the anode catalyst to the membrane in fuel cell.