Three-dimensional numerical simulations were carried out for mixed convective flows over a backward-facing step in a rectangular duct. Reynolds number, expansion ratio and aspect ratio were kept constant at Re = 125, ER = 2 and AR = 16, respectively. Heat flux at the wall downstream of the step was kept uniform, while other walls were kept at adiabatic condition. Effect of the inclination angles, theta(1), theta(2), was the main objective in this study. It was found that when theta(1) was varied, the effect of buoyancy became prominent at theta(1) = 0 degrees, 180 degrees: while the effect was relatively small for the two horizontal cases (theta(1) = 90 degrees, 270 degrees). However, there was still small difference between theta(1) = 90 degrees and 270 degrees in the region immediately after the step where the flow was relatively slow. When theta(2) was varied, flow and thermal fields became asymmetric about the duct centerline, except when theta(2) = 0 degrees, 180 degrees. The maximum Nusselt number, which appears symmetrically near the side walls in pure forced convection cases, was obtained at only one location close to the lower side wall.