Inorganic fiber such as carbon fiber is modified by a surface treatment with 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, an aliphatic group, an alicyclic group or an aromatic group, with the proviso 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, with the proviso that R.sup.2 and R.sup.3 may conjointly form a ring. The inorganic fiber thus modified is useful for reinforcement of resins.