Studying dynamic characteristics of proton exchange membrane fuel cells (PEMFCs) during startups/shutdowns is of great importance to proposing strategies to improve fuel cell performance and durability. In this study, internal current during startup and shutdown processes in PEMFC is investigated, and effects of gas supply/shutoff sequences and backpressure are analyzed by measuring local current densities and the cell voltage in situ. The experimental results show that when reactants were fed/shut off, internal current occurs and variation patterns of local current densities along the flow channel are different. During startups, local current densities in the downstream drop to negative values and internal current can be eliminated when air is first supplied into the cell. While during shutdowns, the results show that negative currents occur in the upstream, and if hydrogen is shut off first, all local current densities remain constant at zero, indicating the effectiveness of gas shutoff sequence in eliminating/mitigating internal current in PEM fuel cells. Further experimental results show that the magnitude of internal current increases with the pressure difference between the anode and the cathode.