Mode switching between fuel cell and water electrolyzer of a unitized regenerative fuel cell alters two-phase flow dynamics in the channels. To fully understand the mode switching, the observation of two-phase flow is necessary. In this work, experiments are conducted to investigate the two-phase flow in the flow field of a unitized regenerative fuel cell during mode switching. A transparent window is assembled at the oxygen side to allow a direct view of the serpentine flow field. The two-phase flow are captured by a high-speed camera. Water has a significant effect on the mode switching. The switching from water electrolyzer to fuel cell is difficult because of the flooding problem. High temperature causes insufficient water in water electrolyzer mode without water supply and membrane dehydration in fuel cell mode. The voltage in fuel cell mode decreases more rapidly with fuel cell current density during mode switching.