The first numerical study to investigate the flow and heat transfer over a BFS using nanofluids was done by Abu-Nada . The Reynolds number and nanoparticles volume fraction used were in the range of 200 ≤ Re ≤ 600 and 0 ≤ Ø ≤ 0.2, respectively, for five types of nanoparticles which are Cu, Ag, Al2O3, CuO, and TiO2. He reported that the high Nusselt number inside the recirculation zone mainly depended on the thermophysical properties of the nanoparticles and it is independent of Reynolds number. Numerical analysis of forced and mixed convection over horizontal and vertical BFS in a duct using different nanofluids was conducted by Mohammed et al. [6-7]. The effects of Reynolds number (in the range of 75 ≤ Re ≤ 225), temperature difference (in the range of 0 ≤ ΔT ≤ 30 °C), and nanofluid type (such as Au, Ag, Al2O3, Cu, CuO, diamond, SiO2, and TiO2) were investigated on the fluid flow and heat transfer characteristics. It is found that a recirculation region developed straight behind the BFS which appeared between the edge of the step and few millimeters before the corner which connects the step and the downstream wall. In the few millimeters zone between the recirculation region and the downstream wall, a U-turn flow was developed opposite to the recirculation flow which is mixed with the unrecirculated flow and travels along the channel. It is inferred that Au nanofluid has the highest maximum peak of Nusselt number, while diamond nanofluid has the highest minimum peak in the recirculation region.