In order to investigate the effects of the buoyancy and finite size particles on the heat transfer process in the turbulent thermal boundary layer, we have carried out three simulations in the present paper, a single-phase neutral boundary layer, an unstable boundary layer with buoyancy effect, and a multi-phase boundary layer with thousands of finite size particles. This is the first time that particle-resolved direct numerical simulation (PR-DNS) is used in the study of thermal boundary layer. The DNS results show the turbulent statistics as well as the thermal structures. It turns out that both the buoyancy and the finite size particles will dramatically affect the turbulent statistics of the boundary layer. Detailed comparisons between the three simulations reveal that the buoyancy effect reshapes the coherent structures in the boundary layer, while finite size particles mainly induce additional disturbance all over the computational domain. Specifically, the Reynolds shear stress and the wall normal turbulent heat flux are remarkably enhanced in the log region by the effect of buoyancy. On the other hand, the finite size particles cause remarkable increment of velocity fluctuations all over the boundary layer, while have the effect of stabilizing temperature fluctuation near the wall. (C) 2017 Elsevier Ltd. All rights reserved.