Current overshoot resulting from a sudden decrease in the cell voltage during start-up process could cause performance fluctuation and reduced durability in proton exchange membrane (PEM) fuel cells. In this study, transient responses of local current densities and high frequency resistance (HFR) of the cell are measured in situ. The experimental results show that when the cell starts up under potentiostatic mode, both the magnitude of current overshoot and the fluctuation of HFR decrease with the increase of anode humidity. A dimensionless current overshoot is defined to describe the magnitude of current overshoot during start-ups. Experimental results show that dimensionless current overshoots are very different at different locations along the flow direction under different anode humidification. When unsaturated hydrogen is fed into the cell, dimensionless current overshoot increases along the flow direction; while as anode gas is fully- or over-humidified, the dimensionless current overshoot decreases along the flow direction. Further experimental results show that the magnitudes of current overshoot are significantly reduced when a linear start-up strategy is used, indicating that the linear start-up strategy is effective in alleviating current density overshoots in PEM fuel cells during start-ups. Copyright (c) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.