Uniformly sized, fine particles of a polymer are produced by:(1) mixing (B) a nonionic-organic compound being liquid at room temperature, having a solubility in water at 25.degree. C. of 0.01 wt. % or larger and being compatible with (A) an oil-soluble, ethylenically unsaturated monomer, with the monomer (A) in an amount of 0.5 to 300 wt. % based on the monomer (A),(2) then adding the mixture of (A) and (B) to an aqueous dispersion of (C) particles of a seed polymer so as to make the mixture of (A) and (B) absorb onto the paricles (C) and(3) polymerizing the mixture of (A), (B) and (C) in the presence of an oil-soluble initiator for the polymerization to obtain said uniformly sized, fine particles.