This work establishes three-dimensional transient numerical models of proton exchange membrane fuel cells (PEMFCs) with different cathode flow field designs. Exactly how flow field design and voltage loading affect the transient characteristics of the PEMFCs are examined. When the operating voltage instantaneously drops from 0.7V to 0.5V, the electrochemical reactions increase. To ensure sufficient oxygen supply for the fuel cell, the oxygen mass fractions are high in the cathode gas diffusion and cathode catalyst layers, causing overshoot of the local current density distribution. When the operating voltage suddenly increases from 0.5V to 0.7V. the electrochemical reactions become mild, and furthermore the oxygen mass fraction distribution becomes low, leading to undershoot of the local current density distribution. The transient response time required to reach the steady state for the parallel flow field with baffle design is longest in the event of overshoot or undershoot among the different cathode flow field designs. The overshoot or undershoot phenomena become more obvious with larger voltage loading variations. Moreover, the transient response time for the Z-type flow field with baffle design is longer than for the Z-type flow field design. (C) 2010 Elsevier B.V. All rights reserved.