A novel process is proposed for the preparation of vinyl chloride based resins modified with a polymer latex, according to which the latex polymer is absorbed so completely and uniformly on the polyvinyl chloride resin that the effect of the modification of the resin properties is remarkably enhanced along with no adverse effects on the particle size distribution of the resin product and no effluence of the latex into the waste water discharged in the dehydration of the aqueous polymerizate slurry after completion of the polymerization. The process of the invention comprises introducing the modifier polymer latex into the polymerization mixture only when the monomer conversion is in the range from 30 to 95% and introducing an inorganic fine powder such as fume silica, bentonite and diatomaceous earth into the polymerization mixture at the same time as or before the introduction of the modifier polymer latex.