Solid oxide fuel cells (SOFCs) operate at high temperature which enables the direct methane steam reforming but results in high thermal impact. In this research, the developments of thermal impacts (thermal stresses and strains) of solid electrolyte and porous electrodes are investigated in a single direct methane steam reforming SOFC by numerical simulations. To understand thermal impact mechanisms, the heat sources and sinks due to a set of temperature dependent chemical and electrochemical reactions are modelled for predictions of temperature distributions. It is identified from model simulations that the endothermic reactions of methane steam reforming, which are overall dominant, play the key role in improving the thermal loads to the cell. The temperature reductions are developed from a rate of −42.2 K/cm at the inlet to −1.98 K/cm at the centre of the cell. This leads to a maximum thermal stress of 1867.6 MPa generated in solid electrolyte closing to the inlet and 1770.2 MPa at the centre of the cell, associated with a rate of −19.48 MPa/cm at inlet to −3.28 MPa/cm at the centre. It is identified that high operation voltage results 3.1% decrease in thermal stress when the cell operates from 0.4 V to 0.7 V.