This paper concerns with a similarity solution for mixed-convection boundary layer copper-water nanofluid flow over a horizontal flat plate. Appropriate similarity variables are used to convert the Governing PDEs to ODEs and the resultant equations with the nanofluid properties relations are discretized and solved simultaneously using finite-difference Keller-Box method. The effects of change in plate temperature, the volume fraction of nanoparticles, and the mixed-convection parameter, on friction coefficient, Nusselt number and velocity and temperature profiles are investigated. The results show that, the Nusselt number increases as the mixed-convection parameter and the volume fraction of nanoparticles increases. This enhancement is about 10 percent for the nanofluid with 4% volume fraction of nanoparticles, compared with the pure water. In this range, moreover, the friction coefficient parameter increases about 20 percent. However, the lower the mixed-convection parameter is, the effect of nanoparticles on the friction coefficient increment is more. The results also illustrate that the effect of the surface temperature on the increment of Nusselt number and on the reduction of friction coefficient is more considerable in higher mixed-convection parameter and volume fraction of nanoparticles. Also by increasing surface temperature, the temperature of nanofluid decreases at any surface distance.