Laminar mixed convection flow over a 2D horizontal microscale backward-facing step (MBFS) placed in a duct is numerically investigated. The governing equations along with the boundary conditions are solved using the finite volume method (FVM). The upstream wall and the step wall are considered adiabatic, while the downstream wall is heated by uniform heat flux. The straight wall of the duct is maintained at a constant temperature that is higher than the inlet fluid temperature. Different types of nanoparticles such as Al2O3, CuO, SiO2 and ZnO, with volume fractions in the range of 1-4% are used. The nanoparticles diameter was in the range of 25 nm <= d(p) <= 70 nm. The expansion ratio was 2 and the step height was 0.96 mu m. The Reynolds number was in the range of 0.05 <= Re <= 0.5. The results revealed that the Nusselt number increases with increasing the volume fraction and Reynolds number. The nanofluid of SiO2 nanoparticles is observed to have the highest Nusselt number value. It is also found that the Nusselt number increases with the decrease of nanoparticle diameter. However, there is no recirculation region was observed at the step and along the duct. (C) 2012 Elsevier Ltd. All rights reserved.