Surface accumulation method, called the Hwang-Balluffi method [J.C.M. Hwang, J.D. Pan and R.W. Balluffi, J. Appl. Phys., 50(3), 1979, 1339], was applied to measure the grain-boundary diffusion of Ag at low temperatures (413 and 428 K) in a nanocrystalline Cu film. Ag atoms from the Ag layer diffused through the copper nanocrystalline film along the grain boundaries to the opposite surface (i.e. the accumulation surface) where they spread out by rapid surface diffusion and accumulated. The rate of accumulation was detected by Auger Electron Spectroscopy (AES). It was shown that the results are not sensitive to the supposition whether accumulation takes place in one or two monolayers of the surface. TEM observations have been made in the same time before and after heat treatment to check the stability of the nanostructure.