The conjugate heat transfer of impingement jet is investigated in order to better reveal its underlying principles. The study is carried out basing on a validated CFD model of free-air-jet that discharges from a round nozzle and impinges perpendicularly onto a solid plate with uniform heat flux boundary condition on heated surface. Different operating parameters including plate thickness, plate material, jet Reynolds number and nozzle diameter are investigated. It is observed that these parameters alter the Nusselt number and the thermal condition at the fluid-solid interface. Results show that thermal conjugate effect redistributes the boundary heat flux and convert the thermal boundary from uniform heat flux condition into approximately isothermal condition. This is driven by the non-uniform distribution of thermal convection resistances on impinged surface. The strength of heat redistribution is related to both the thermal conduction resistance and thermal convection resistance. Heat flow in the conjugate heat transfer process of impingement jet is illustrated by using heat transfer network methodology, which helps better understand the process of heat redistribution and thermal boundary altering. Another focus of present work is the discrepancy of Nusselt numbers between conjugate case and non-conjugate case. It is found that the conjugate effect leads to the decay of Nusselt number. This is interpreted from the viewpoint of field synergy principle and it turns out to be a consequence of the degradation in synergy between thermal field and flow field due to thermal conjugate effect. (C) 2016 Elsevier Ltd. All rights reserved.