Ostwald ripening of precipitate particles in Cu–Co alloys was investigated by electron microscopic observation. Radii of individual particles were directly measured from their strain contrasts in the electron microscopic images for the alloys containing 1–4 at%Co, and the variation of the mean particle size and the distribution of the relative particle size were obtained as functions of the aging time at 873 K. A systematic change in the distribution with the aging time was observed, which suggested that the observed coarsening process did not reach the steady state as assumed in the theories of LSW and Ardell. The shape of the distribution of particle size is sharp and symmetrical around the mean particle size in the early stage of aging, while it becomes broad and asymmetrical with increase in the aging time. This change in the size distribution with the aging time differs from the expectation from the rate equation for individual particle growth in the LSW theory and from the experimental results of some Ni-based alloys. The size distribution observed at the late stage of aging was similar, especially in the high concentration alloy, to that predicted by Ardell who took into account the effect of the volume fraction of precipitates. The size distribution seemed to depend on the mean concentration of the alloy, namely on the volume fraction. However, the growth rate of the mean particle radius is almost independent of the mean concentration in contradiction to Ardell’s prediction, although the growth rate follows the (aging time)^1⁄3 law for diffusion-controlled coarsening.