A heat resistant polymer composition having improved processability and oxidative deterioration resistance which comprises a blend of a polyphenylene oxide copolymer and a polymer prepared by radical polymerization of one or more kinds of monomers having at least one vinyl group in the molecule, the polyphenylene oxide copolymer having a polyphenylene ether structure derived from a phenol compound and containing nitrogen-containing monomer units derived from at least one of the compounds of the general formulas: ##STR1## wherein R.sup.5 to R.sup.11 are hydrogen, a hydrocarbon group or a substituted hydrocarbon group, provided that R.sup.5 and R.sup.6, R.sup.8 and R.sup.9, and R.sup.10 and R.sup.11 can be joined to form a substituted or nonsubstituted aromatic ring.