A thermoplastic resin composition which comprises 100 parts by weight of a composition comprising 95-5 % by weight of at least one polyolefinresin (A) selected from homopolymers of ethylene or .alpha.-olefin or copolymers thereof and these homopolymers or copolymers modified with a polyfunctional compound (E) and/or an unsaturated monomer (L) and 5-95 % by weight of at least one polyphenylene-ether-resin (B) selected from polyphenylene ether, modified polyphenylene ether with the above (E) and/or (L) and a composition comprising the polyphenylene or modified polyphenylene ether and an aromatic vinyl polymer resin (M) and 0.001-10 parts by weight of a dinitrodiamine (D) of the following formula (I). ##STR1## (wherein X represents a divalent chain aliphatic group, a cyclic aliphatic group or an aromatic group which may contain a halogen or an oxygen atom, R.sup.1 represents a hydrogen atom, a chain aliphatic group, a cyclic aliphatic group or an aromatic group and when both of X and R.sup.1 are chain aliphatic groups, the nitrogen atoms may further bond each other to form a ring through X and R.sup.1 ; R.sup.2 and R.sup.3 are independently a hydrogen atom or an alkyl group of 1-12 carbon atoms and R.sup.2 and R.sup.3 may bond to form a ring).