Transient response of Proton Exchange Membrane Fuel Cell (PEMFC) is very crucial for transportation and residential power source subjected to a variable loads. Fully three dimensional transient simulation was performed numerically in order to analyze the dynamic response of a proton exchange fuel cell with single serpentine flow channels. The results show that overshoot is observed when the cell voltage is changed from 0.7 V to 0.5 V and the current density reaches to the steady state value as time is elapsed. Ion conductivity is the highest at the time of the load change and then reach to the steady state value. The same trend is also observed for the current density. When the cell voltage is changed from 0.7 V to 0.5 V. the fuel cell temperature is increased with increasing the heat of formation but the rise in the temperature is not rapid as that in the voltage. Furthermore the lag in heat transfer through the reaction heat and higher ion conductivity at the drop of cell voltage are thought to cause overshoot behavior of the peak of the current density and then current density decreases as the heat transfer increases temperature, which gradually leads to low ion conductivity. (C) 2011 Elsevier Ltd. All rights reserved.