On duct system in industrial plants and air conditioners, free convection is produced by buoyancy forces in the fluid because of the temperature differences between the duct wall and the fluid. As a result, it was indicated that secondary flow vortices in a cross section of the duct occur, and counter-rotating additional vortices arise in a region near the bisector of the bottom wall at high ReRa. In this paper, steady laminar flows with free convection in horizontal rectangular ducts are studied numerically for air (Pr = 0.73) at ReRa =10^5, 8 × 10^5 on high aspect ratio. The effects for flow patterns of additional vortices on axial velocities, temperatures, stream lines of secondary flow, friction losses and Nusselt numbers are presented.