The cobalt nanoparticles have been prepared by reduction of Co^2＋ with hydrazine in ethanol-aqua system at room temperature. It was found that the overall reaction occurs in two stages. The first stage is the reduction of Co^2＋ by N₂H₄ according to 2Co^2＋＋N₂H₄＋4OH^－＝2Co¯＋N₂­＋4H₂O, while the second stage consists of two side reactions, the decomposition of hydrazine (N₂H₄＝N₂­＋2H₂­) and the disproportionation of hydrazine (3N₂H₄＝N₂­＋4NH₃­) catalyzed by the formed cobalt nanoparticles. The reaction for the formation of cobalt is catalyzed by cobalt as an autocatalytic reaction. Accordingly, a small amount of inducing agent, NaBH₄ can be added to initiate the reaction that can then proceed continuously at 288 K. The kinetic equation of the reaction for the reduction of Co^2＋ by hydrazine has been derived by monitoring the volume of gases evolved versus time. It reveals that the reaction orders with respect to Co^2＋, N₂H₄ and Co are 1, 0 and 1, respectively. Thus, the increase of the initial concentration of Co^2＋ and the amount of inducing agent used accelerate the reaction rate and decrease the consumption of N₂H₄ significantly. The activation energy of the autocatalytic reaction was found to be about 89 kJ•mol^－1. Change of initial concentration of Co^2＋ might alter the surface area of Co nanoparticles from 11 to 25 m²/g, corresponding to the particles with average diameters from 60 to 30 nm.

Key words: cobalt nanoparticle, reduction by hydrazine, reaction kinetics, autocatalytic reaction