Because　of　the　stringent　nitrogen　oxides　(NOx)　emissions　legislation　for　heavy-duty　diesel　engines,　the　SCR　(selective　catalytic　reduction)　has　been　investigated　by　many　researchers.　The　objective　of　these　studies　is　to　investigate　the　reasons　why　the　temperature　downstream　of　the　SCR　catalyst　is　not　always　equal　to　that　upstream　with　a　commercial　V2O5　catalyst　applied　in　a　heavy-duty　diesel　engine.　Using　the　energy　conservation　equation　and　SCR　reaction　model,　a　method　for　estimating　the　exhaust　gas　temperature　downstream　of　the　catalyst　was　proposed.　The　SCR　downstream　temperature　was　calculated　and　experimentally　measured　under　the　standard　ESC　(European　steady-state　cycle)　and　different　speeds　and　torques.　The　results　indicated　that　when　the　diesel　engine　operated　under　steady-state　conditions,　the　temperature　downstream　of　the　catalyst　was　similar　to　that　upstream.　Compared　with　the　change　of　the　temperature　upstream　of　the　catalyst,　the　change　of　downstream　is　delayed　under　unsteady-state　conditions.　According　to　the　analysis,　the　main　reason　for　this　result　is　that　temperature　gradients　exist　along　the　substrate　channels,　accounting　for　inter-phase　(gas　solid)　heat　transfer.　The　downstream　temperature　estimation　is　important　for　confirming　the　SCR　catalyst　temperature.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.