Three-dimensional numerical simulation has been performed for flows over a backward-facing step at low Reynolds number in order to investigate the effects of the duct aspect ratio. Numerical results were compared with other investigations and found to agree well with experimental data. Close attention was paid to the distribution patterns of both Nusselt number and the skin friction coefficient on the bottom wall. It was found that an aspect ratio of as large as AR = 16 at least was needed to obtain a 2D region near the centerline at Re = 250. This 2D region became wider for a lower Reynolds number. It was also found that the maximum Nusselt number did not appear on the centerline but near the two side walls in every case.