The engineering model, where the number concentration of particles generated by homogeneous nucleation n* is determined so that the generation rate of monomers G* is equal to their vanishing rate at monomer concentration C*, which corresponds to a critical supersaturated state of nucleation, has been proposed by the present authors (Kousaka and Nomura, Kagaku Kogaku Ronbunsyu, 23, 666-672 (1997)), The cell model, which is familiar in particle dispersed systems, is applied and the solution of unsteady diffusion rate of monomers onto a nucleus is used in the analysis. As a result, we obtain the relation among the parameters as G* = 4 pi r*DC*n*, where D is diffusion coefficient of monomer, and r* is radius of nucleus. The effect of monomer diffusion coefficient on particle generation, which is one of the factors included in the engineering model, is studied for systems where silver particles are generated by a reduction method in the liquid phase. The monomer diffusion coefficient is controlled by changing viscosity of the solution by addition of a thickener, and it is found experimentally that the number concentration of particles generated is inversely proportional to the monomer diffusion coefficient resulting in particle size reduction, which quantitatively support the model.