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.