In this work, a three dimensional laminar mixed convection of nanofluid flow in horizontal concentric annulus is investigated numerically based on single-phase fluid approach. The constant heat flux is applied on the outer cylinder, while the inner cylinder is adiabatic. The annular space is filled with Silver-Water nanofluid. The governing equations are discretized using the finite-volume method and the SIMPLER algorithm was used for resolve the pressure-velocity coupling. The results are presented for the dynamical and thermal fields for a fixed Reynolds number to 800, two Grashof numbers to 105 and 5×105, and the particle volume fraction from zero to 6%. According to the results, the Nusselt number and the bulk and walls temperatures increase with the increasing of the volume fraction and the Grashof number. Also a very good agreement with the results of similar works available in the literature is observed.