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.