Stable nanosize gold and silver particles are spontaneously formed by reduction of aqueous metal salt solutions in the presence of sugar-persubstituted poly(amidoamine) dendrimers (sugar ball). Two sugar balls consisting of generations 3 and 5 are used. The nanoparticles are characterized by absorption spectroscopy, electron microscopy, and FT-IR spectroscopy. In the formation of silver particles, alkaline solution is required. In the process of reduction, characteristic absorption bands appear for the respective metal particles and the intensities increase with reaction time. The sigmoidal curve of intensity vs the reaction time suggests an autocatalytic reaction. The apparent rate constants for the formation of gold particles increase with the concentration of sugar ball, where the rate constants are greater for generation 3 than for generation 5 at the same weight percent of sugar ball. On the other hand, an opposite trend is observed for the formation of silver particles. The average diameters of gold and silver range between 2 and 6 nm and their size distributions are relatively wide.