Numerical simulations are performed to study the transition of the development of thermal boundary layer of air along isothermal heated plates in a large channel. In particular, the aim is to investigate the effects of the channel width on the transition of the flow under various plate temperatures. Realizable k-epsilon turbulence model with an enhanced wall function is employed to obtain the numerical simulations of flow and thermal fields in the channel. The channel width is varied from 0.04 m to 0.45 m and the numerical results of the maximum values of flow velocity, turbulent kinetic energy are recorded along the flow to examine the critical distance of the developing flow. Effects on the transition of the two different types of wall boundary conditions, isothermal and adiabatic, applied to the channel are also examined. The results particularly indicate that the flow transition in the isothermal cases takes later than that in the adiabatic cases. (C) 2014 Elsevier Ltd. All rights reserved.