To comprehensively understand the cold start processes of proton exchange membrane fuel cell (PEMFC) stack which is important for the automotive applications, a three-dimensional multiphase PEMFC stack model is developed in this study. The detailed analysis of the cold start processes shows that for the stacks with more cells, the voltage decreases more slowly due to the lower ice formation rates. The temperature increases faster for a stack with more cells, and a higher temperature can be reached at the end of the cold start process. No apparent difference in voltage exists among the different individual cells in a stack when the reactant gases are evenly supplied to each cell. The temperature in the individual cell in the middle of a stack is higher and more evenly distributed than those on the sides and single cells, due to weakened cooling effect of the bi-polar plate (BP) on the membrane electrode assembly (MEA), and the ice formation rate is also lower in the middle cell. At a lower current density, the ice in the cathode catalyst layer (CL) is formed faster at the section close to the BP, and it is close to the membrane at a higher current density. (C) 2012 Elsevier B.V. All rights reserved.