||Dynamic response is a very important issue in a vehicle powered by a proton exchange membrane fuel cell (PEMFC) due to its frequent load change. In this study, three-dimensional transient simulation was conducted to investigate the dynamic response of a high temperature PEMFC. A stepwise change in the cell voltage was conducted to simulate the sudden load change during vehicle operation. After the load change, the oxygen concentration, ionic conductivity, and local current density were analyzed, and the water transport through the membrane electrode assembly (MEA) was also evaluated to determine the characteristics of overshoot and settling time. The result showed that the overshoot was mainly due to the pre-existing excessive oxygen. At the humidified condition, the overshoot increased with increasing relative humidity due to higher ionic conductivity and electrochemical reaction rate. In addition, a stepwise mass flow control strategy for the reactants decreased the overshoot and settling time by decreasing the amount of oversupplied oxygen immediately after the load change. The settling time for the stepwise mass flow control was approximately 30% lower than that for the constant mass flow rate control.