The dynamic characteristics of the proton exchange membrane (PEM) fuel cell are of great importance in its design and applications. In this paper, the dynamic process of stack voltage is analyzed when the current is step inputted. We discuss the process from the following four aspects: voltage variation rate, initial value of dynamic voltage, time to reach steady state and dynamic resistance factor. The analysis results show that the dynamic process of stack voltage responding to current step-up is different from that to current step-down. Additionally, the operating current values also have significant influence on the dynamic characteristics of stack voltage. (c) 2010 Elsevier Ltd. All rights reserved.