Cu nanoparticles approximately 30 nm in diameter were electrochemically formed by liquid-phase reduction (i.e., electroless deposition) using hydrazine as a reducing agent and dispersing barely soluble CuO particles in aqueous solution at 353 K. The addition of gelatin into the reaction suspension prevented the coagulation of Cu nanoparticles and also sufficiently suppressed particle growth. Direct reduction of CuO particles did not occur due to the adhesion of gelatin on the surface of CuO particles, and Cu nanoparticles were deposited by the reduction of Cu2+ ions dissolved in solution from CuO particles. The solubility of CuO particles was extremely small (of the order of 10-18 mol dm-3 at 353 K) in aqueous solution of pH 12, which regulated the size of Cu particles. Cu nanoparticles approximately 300 nm in diameter were obtained at 323 K, while Cu nanoparticles of two different sizes (less than 50 nm and about 300 nm) were obtained at 333-353 K. The proportion of the smaller particles increased with increasing reaction temperature, and the smaller particles were easily separated by centrifugal classification without aggregation. The formation mechanism of nanoparticles was discussed from the viewpoint of thermodynamics with in situ monitoring of Cu deposition and immersion potential.