PURPOSE: To obtain a high-quality polymer having excellent storage stability, thermal stability and plasticizer-absorbability, by adding a liquid dispersion composed of an antioxidant, a dispersing agent and water to a polymerization system in the suspension polymerization of a vinyl monomer.

CONSTITUTION: Vinyl chloride or vinyl monomers containing vinyl chloride are polymerized by suspension polymerization in an aqueous medium. The polymerization is carried out by mixing the polymerization system with a liquid dispersion composed of an antioxidant {preferably having a melting point of 30W100°C, e.g. triethylene glycol bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl) propionate}, a dispersing agent (preferably a water-soluble polyvinyl alcohol having a saponification degree of 60W98% and a polymerization degree of 600W3,000) and water. The amount of the liquid dispersion is usually 0.001W1.0wt.% based on the monomer.