The contributions of phonon and electron transport to the thermal conductivities of Cu and Ag nanowires are studied theoretically. The effects of surface and grain boundary scatterings are involved. The embeded atom method is employed to express the interatomic potential of nanowires. While the molecular dynamic simulation and Green-Kubo formulation are used to obtain the lattice thermal conductivity, a model derived from Boltzmann transport equation and the Wiedemann-Franz relation are used to calculate electronic thermal conductivity. In addition, diffuse mismatch model is used to calculate thermal resistance of grain boundary to modify the lattice thermal conductivity, meanwhile, Mayadas-Shatzkes model is used to consider the influence of grain boundary scattering on the electronic thermal conductivity. By coupling the lattice and electronic thermal conductivity, the effective thermal conductivity of nanowire is obtained. On this base, the influences of size and temperature are analyzed. It turns out that Cu and Ag nanowires have a similar tendency in the thermal conductivity. The contribution of electron transport to the thermal conductivity of nanowire is dominated, but the contribution of phonon transport cannot be ignored on the nanoscale. The thermal conductivity of nanowire decreases due to the grain boundary scattering. And it decreases with temperature increasing or size decreasing. The contribution of phonon transport becomes more important in the case of smaller size.