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Silver nanoparticles were prepared by the reduction reaction of silver nitrate solution with sodium hypophosphite in polyvinylpyrrolidone solution. The silver sol was coagulated by adding ammonia solution. Some technical conditions, such as pH value, ammonia concentration, ammonia dropping velocity and stirring rate were tested in order to get silver nanoparticles by filtrating silver sols. The results showed that the separation of solid silver nanoparticles with liquid solution was improved and the silver nanoparticles with different sizes were prepared using this method. The silver powders with particle size of 10 nm~20 nm were successfully separated from the silver sol by adding mixed solution of flocculent and organic alkaline agents. This process is simple and ordinary, needs some simply apparatuses only and consumed less energy with a high silver production capacity. Therefore, it is feasible for industrialized preparation of silver nanoparticles. The products were analyzed by transmission electron microscope (TEM) and X-ray diffraction (XRD) and shown that the particles were pure metal silver with high dispersible spherical form and diameters from 10 nm to 40 nm.