A thermoplastic resin is stabilized by incorporating therein a dinitrodiamine compound represented by the formula of ##STR1## wherein X is a divalent aliphatic, alicyclic or aromatic group which may contain halogen or oxygen in the group, R.sup.1 is hydrogen or an aliphatic, alicyclic or aromatic group, provided that two nitrogen atoms linking through X may further link through R.sup.1 when both X and R.sup.1 are the aliphatic groups, and R.sup.2 and R.sup.3 independently of one another are each hydrogen or an alkyl of 1 to 12 carbon atoms, provided that R.sup.2 and R.sup.3 may conjointly form a ring. The composition thus blended is improved in thermal processing stability. Further, a modified polyolefin resin is improved in mechanical properties, particularly in impact strength, by the incorporation of the dinitrodiamine compound.