Dendrimer-encapsulated Co nanoparticles (G6-OH(Co60)) have been synthesized through the complexation of Co2+ cations to the internal tertiary amine of sixth-generation hydroxyl-terminated poly(amidoamine) dendrimers followed by reduction by both sodium borohydride and ammonia borane in aqueous solution. The highly dispersed G6-OH(Co60), in which the average diameter of Co nanoparticles is approximately 1.6±0.4 nm, have been confirmed by TEM analysis. The catalytic activity of G6-OH(Co60) for the hydrolysis of ammonia borane has been studied in aqueous solution at different reaction temperatures, demonstrating a high catalytic performance.