Cu nanoparticles were prepared in di(ethylene glycol) by a reduction reaction of Cu (II) acetate precursor to metallic Cu. The size and morphology of the synthesized particles were studied in dependence of the concentration of the starting compound and the temperature conditions of reaction were varied to determine the correlation with the size and morphology of the synthesized particles. The morphology and size of the resulting copper (I) oxide as an intermediate product and metallic Cu particles as a final product are strongly dependent on the concentration of the starting compound, thus indicating differences in the mechanism of the reduction reaction and, consequently, the mechanism of particle formation. At low concentrations (0.01 and 0.1 mol/L), an organo-metallic copper complex intermediate forms crystalline 10-100 nm thick and up to 10 microm long nanowires organized in dendritic spheres with a diameter of 5-50 microm, which further transform into Cu2O. Cu-di(ethylene glycolate) complex has an as yet undescribed crystalline structure. At a high precursor concentration (1 mol/L), the intermediate forms partly amorphous and partly crystalline Cu20. The reduction of Cu2O to metallic Cu takes place between 190-200 degrees C. The smallest average particle size (100 nm) and the narrowest particle size distribution was obtained at a Cu (II) acetate concentration of 0.1 mol/L.