Recent studies have shown that Soret diffusion has strong impact on the laminar flame speed of hydrogen/air mixture while it has little influence for hydrocarbon fuels at normal temperature and pressure. However, it is not clear whether the same conclusions hold under engine-relevant conditions. This is investigated in the present study. A series of premixed spherical flames propagating at high temperatures and pressures are simulated for different fuels, and the corresponding laminar flame speeds with and without considering Soret diffusion are obtained. It is found that for hydrogen/air mixture, Soret diffusion has much stronger influence on burning rate at engine-relevant conditions than at normal temperature and pressure. However, for hydrocarbon fuels like methane and iso-octane, Soret diffusion still has negligible effects on the burning rate even under engine-relevant conditions. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.