In Cu–Ni–Si alloy, the strength can be improved by finely dispersing a Ni–Si based compound as a precipitate into the Cu parent phase by heat treatment. In order to investigate the effect of the precipitate on strengthening, quantitative evaluation of the size distribution and dispersion state is necessary. In this paper, transmission electron microscopy, small angle X–ray scattering, small angle neutron scattering, and atom probe tomography were utilized to analyze this Ni–Si precipitated phase. Small angle X–ray and neutron scattering results showed that the precipitated phase gradually became coarser as the aging heat treatment temperature increased. Atom probe tomography and small–angle scattering provided complementarily measurements of the diffusion layer at the interface between the Cu parent phase and the precipitated phase.