Knowledge　concerning　the　complicated　changes　of　mass　and　heat　transfer　is　desired　to　improve　the　performance　and　durability　of　unitized　regenerative　fuel　cells　(URFCs).　In　this　study,　a　transient,　non-isothermal,　single-phase,　and　multi-physics　mathematical　model　for　a　URFC　based　on　the　proton　exchange　membrane　is　generated　to　investigate　transient　responses　in　the　process　of　operation　mode　switching　from　fuel　cell　(FC)　to　electrolysis　cell　(EC).　Various　heat　generation　mechanisms,　including　Joule　heat,　reaction　heat,　and　the　heat　attributed　to　activation　polarizations,　have　been　considered　in　the　transient　model　coupled　with　electrochemical　reaction　and　mass　transfer　in　porous　electrodes.　The　polarization　curves　of　the　steady-state　models　are　validated　by　experimental　data　in　the　literatures.　Numerical　results　reveal　that　current　density,　gas　mass　fractions,　and　temperature　suddenly　change　with　the　sudden　change　of　operating　voltage　in　the　mode　switching　process.　The　response　time　of　temperature　is　longer　than　that　of　current　density　and　gas　mass　fractions.　In　both　FC　and　EC　modes,　the　cell　temperature　and　gradient　of　gas　mass　fraction　in　the　oxygen　side　are　larger　than　that　in　the　hydrogen　side.　The　temperature　difference　of　the　entire　cell　is　less　than　1.5　K.　The　highest　temperature　appears　at　oxygen-side　catalyst　layer　under　the　FC　mode　and　at　membrane　under　a　more　stable　EC　mode.　The　cell　is　exothermic　all　the　time.　These　dynamic　responses　and　phenomena　have　important　implications　for　heat　analysis　and　provide　proven　guidelines　for　the　improvement　of　URFCs　mode　switching.