Three-dimensional numerical simulation of steady-state laminar forced convection flow of a CuO/water nanofluid and heat transfer in a duct having a microscale backward-facing step (MBFS) are presented in this study. The study was conducted for determining the effects of nanoparticle volume fraction on the flow and heat transfer characteristics. The Reynolds number ranged from 100 to 1000. The step height and inlet height of the duct were 600 mu m and 400 mu m, respectively. The duct expansion ratio was 2.5. The downstream wall was subjected to a constant and uniform heat flux, whereas the other walls were insulated. The nanoparticle volume fraction varied from 1.0% to 4.0%. The Nusselt number and Darcy friction factor were obtained for each nanoparticle volume fraction. Plots of velocity streamlines were analyzed. It was found from the results of numerical simulation that the Nusselt number increases with increasing nanoparticle volume fraction and Reynolds number. The nanoparticle volume fraction does not exert any substantial effect on the Darcy friction factor and the length of the recirculation zone. Moreover, the performance evaluation criterion (PEC) was analyzed for nanoparticle volume fractions of 1.0%, 2.0%, 3.0%, and 4.0% of CuO. It was obtained that the volume fractions of 4.0% has the highest PEC in terms of heat transfer. It was obtained that while heat transfer for nanoparticle volume fraction of 30% and 4.0% the friction factor is superior for nanoparticle volume fraction of 1.0% and 2.0% due to the PEC number.