A facile way to synthesize nanometer-sized polymer (polypyrrole, PPy) particles is explored on the basis of the formation of complexes between water-soluble polymers and metal cations in aqueous solution. The metal cation is used as an oxidizing agent to initiate the chemical oxidation polymerization of the corresponding monomer, and the water-soluble polymer effectively provides a steric stability for the growth of polymer nanoparticles during the polymerization process. Light-scattering analyses are performed to give insight into the behavior of the complexes in aqueous solution. In addition, major physical parameters affecting the formation of polymer nanoparticles are investigated, including hydrodynamic radius, radius of gyration, shape factor, and viscosity. By judicious control of these parameters, PPy nanoparticles with narrow size distribution can be readily fabricated in large quantities. It is also possible to control the diameter of the nanoparticles by changing critical synthetic variables. Importantly, PPy nanoparticles of approximately 20-60 nm in diameter can be prepared without using any surfactants or specific templates; this novel strategy offers great possibility for mass production of polymer nanoparticles.