A method for producing a modified polyphenylene oxide which comprises polymerizing 69 to 25 parts by weight of a mixture of 97 to 45% by weight of a styrene monomer and 3 to 55% by weight of a polar vinyl mononer at 90.degree. to 190.degree. C. using 0.3 to 10 parts by weight of a radical initiator in an aqueous dispersion containing a dispersion stabilizer alone or in combination with at least one of a surfactant and an adhesion-preventing agent in the presence of 31 to 75 parts by weight of a polyphenylene oxide having a unit structure of the formula, ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each a hydrogen or halogen atom, or a hydrocarbon, substituted hydrocarbon, cyano, hydrocarbonoxy, substituted hydrocarbonoxy, nitro or amino group.