With　the　occurrence　of　reversible　electrochemical　reactions,　mode　switching　considerably　affects　the　electric　performance　of　unitized　regenerative　fuel　cells　(URFCs)　owing　to　the　complicated　mass　and　heat　transfer.　Although　limited　researches　have　been　done,　no　such　studies　on　mass　and　heat　transfer　through　a　three-dimensional　view　are　envisioned　during　mode　switching.　A　three-dimensional　full-cell　model　was　developed　and　validated　to　study　the　dynamic　characteristics　of　a　proton　exchange　membrane-based　URFC　during　mode　switching.　Mode　switching　was　performed　by　changing　operation　voltage　from　0.60　to　1.65　V.　Results　showed　that　species　and　heat　transfer　affect　the　electric　performance　of　the　cell　during　mode　switching,　especially　through　the　third　dimensional.　Local　water　starvation　occurs　on　oxygen　side　catalyst　layer　and　thus　results　in　slight　reduction　on　current　density　and　hydrogen　generation.　Restricted　to　heat　transfer　capacity　through　ribs,　heat　transfer　process　adds　total　response　time　in　URFCs.　Heat　flux　and　surface　heat　transfer　coefficient　are　forecasted　on　the　hydrogen　and　oxygen　sides.　A　total　time　of　4　seconds　is　essential　for　URFC　reaching　a　new　relative　balanced　state.