The resistivity of single-layered thin copper films with thickness of 17-124 nm, is studied as a function of the temperature and grain diameter. The resistivity of both as-deposited and 500 K annealed films is found to increase with decreasing film thickness. Analysis has shown that the grain-boundary scattering is the dominant contribution and the surface scattering cannot be the cause of the excess resistivity of both as-deposited and 500 K annealed films. The average reflection coefficient R of the electrons scattered by the grain boundaries is found to be 0.38 for both as-deposited and 500 K annealed films over the whole temperature and thickness range studied.