One promising approach to improving thermoelectric energy conversion is to use nanostructured interfaces that enhance Seebeck coefficient while reducing thermal conductivity. Here, we synthesized Au-Cu2Se core-shell nanoparticles with different shell thicknesses by controlling the precursor concentration in solution. The Au-Cu2Se core-shell nanoparticles are about 37-53 nm in size, and the cores of the nanostructures are composed of Au nanoparticles with sizes of ∼11 nm. The effect of shell thickness on the thermoelectric properties of core-shell nanocomposites is investigated after sintering the core-shell nanoparticles into pellets using the spark plasma sintering (SPS) technique. The power factor was optimized by the synergetic effect of the improvement of Seebeck coefficient by energy filtering in the Au/Cu2Se interface and the effective tuning of carrier concentration by Ohmic contact in the interface. The lattice thermal conductivity of core-shell nanocomposites is reduced by coherent phonon scattering, which is caused by the wavelike interference of phonons due to the phase shift in the core-shell interface. The highest ZT value of 0.61 is obtained at 723 K for Au-Cu2Se core-shell nanocomposite with a shell thickness of 21 nm, which is higher than that of pure Cu2Se nanocomposite or a mixture of Au and Cu2Se particles.