A specialized polymeric peroxide is copolymerized with vinyl type monomers in the presence of organic liquid, thereby obtaining a solution of a copolymer having peroxy bonds therein.This solution is admixed with vinyl type monomers which are different in composition from the ones used in the first copolymerization, and the resultant mixture is subjected to block copolymerization.The obtained block copolymer solution is admixed with an organic liquid which exert solubility to one of polymers of the vinyl type monomers which are employed in the first or second copolymerization, whereby a non aqueous system liquid dispersion is obtained.This dispersion contains a block copolymer in a high concentration and it also is splendid in uniformity of dispersion and dispersion stability.