The effect of several forms of suspension polymerized poly(vinyl chloride) particles on mechanochemical block copolymerization in the solid poly(vinyl chloride)-styrene-sodium dodecyl sulfate solutions has been studied by ultrasonic irradiation at 60-degrees-C. The block copolymerization of styrene was initiated by free radicals produced from the poly(vinyl chloride) particles by ultrasonic waves. The rates of copolymerization increased with increasing the additional amount of the solid poly(vinyl chloride), the porosity, and the average diameter of the grain particles. In particular, the influence of the average diameter was much larger than that of the porosity. When the porosity and the average diameter were increased, the rates of decrease in the viscosity-average degree of polymerization of the degraded poly(vinyl chloride) were much increased. In addition, the changes in the composition of the block copolymer and homopolymers in the reaction products were obtained.