Experimental and theoretical studies are reported on the growth of metallic particles during the reduc-tion of metallic salts by borohydride. The increase in reaction rate by the increase in temperature or by the choice of the metallic salt which gives higher reduction rate shows a tendency of reducing the average size of particles formed. A model equation, called General Dynamic Equation, which considers all the terms contributing particle formation and growth processes including chemical reaction has been numerically solved by the method of moment transformation with other necessary equations. the solutions successfully support the experimental observations and also make the growth mechanism clear:more nuclei are formed at higher reaction rate and as r result, there remain fewer monomers available for the subsequent condensation on the nuclei. In this system coagulation of particles has been shown not to be so important in determining the size of growing particles.