Water management in a fuel cell is essential to ensure cell performance and life. In this study, a special single cell was designed for the purpose of detecting liquid water and water vapor simultaneously. The major difference between our design and traditional flow field designs is the fact that the anode and cathode channels were shifted sideways, so that they do not overlap in the majority of the active areas. The liquid water is measured by using neutron radiography located at the National Institute of Standards and Technology. The water vapor is measured by the 20 relative humidity sensors embedded in the anode and cathode flow field plates. The effects of the relative humidity and stoichiometry of the cathode inlet on relative humidity distribution in the channels and on water accumulation in the gas diffusion layers (GDLs) were investigated in this study. The liquid water accumulation at steady-state was calculated by using imaging mask techniques and least-squares method. The transient behavior of water transport was detected and recorded when a step load change was applied on the cell. It is demonstrated that liquid water tends to accumulate in the gas diffusion layers under the rib. Moreover, the transient behavior of liquid water transport in the GDL and the relative humidity distribution in both the anode and cathode channels at different operating conditions are discussed.