Numerical analyses were performed for mixed-convection heat transfer in a square flow channel with uniform wall temperature. Three-dimensional governing equations were solved for Re = 100, Pγ = 0.72 and Gγ/Re² = 0∼50 by finite volume and pressure correction methods. The local flow and thermal development were examined across the cross section and along the flow direction. The effects of Gγ/Re² were shown for the local behavior of Nusselt number and the position of the peak. The development of buoyancy-driven roll generates a three-dimensional reverse flow and prevents the main flow in the thermal developing region. Numerical analysis was also performed for the square channel with an adiabatic entry channel.