Copper nanoparticles have been prepared by reduction of Cu(II) ions in aqueous and non-aqueous solutions. The time at which elemental Cu was formed from starting 0.016M Cu solutions decreased from 10 hours to only 6 minutes when the concentration of hydrazine was increased from 0.059M up to 0.7M, respectively. The corresponding average crystallite size decreased from 30nm down to 16nm. The complete reduction of in ethylene glycol was achieved after 30 seconds when a NaOH/Cu mole ratio of 50 was used. The average crystallite size was estimated at 25 nm. In both approaches, the Cu-forming reaction involved the formation of intermediate Cu2O, which underwent dissolution and subsequent reduction into elemental Cu. UV-vis measurements evidenced the formation of nanosize Cu crystals. The surface plasmon resonance band was blue-shifted after intensive sonication suggesting a decrease in crystal size during the preparation of the nanofluid in ethylene glycol.