The effect of type of turbulence model and nanofluid on the heat transfer and fluid flow in a horizontal narrow rectangular duct is numerically studied under constant wall heat flux boundary condition. Numerical study is carried out using ANSYS Fluent 17.0 software. Examined parameters are the type of turbulence model, the type of nanofluid, the volume fraction of nanoparticle in nanofluid, and the Reynolds number. Three different k-epsilon and four different k-omega turbulence models are employed. Aluminum oxide Al2O3-water and copper oxide CuO-water are used as nanofluids. Volume fractions of nanoparticles used are 0%, 0.1%, 0.5%, 1%, 2% and 4%. Reynolds number changes from 3 x10(3) to 50 x10(3). Results showed that k-omega standard turbulence model with low Reynolds number correction gives better result. It is seen that both the type and the volume fraction of nanoparticle in nanofluid affect heat transfer and pressure drop. Using Al2O3 and CuO nanoparticles in water increases thermal performance. It is found that the performance factor of CuO-water nanofluid is better than that of Al2O3-water nanofluid. It is seen that using turbulent fully developed flow correlations derived for circular ducts may end up with incorrect results for the flow in two-dimensional rectangular duct.