A naphthalene of the following formula (1) having at least two allyl or propenyl groups ##STR1## wherein each G represents a hydrogen atoms or ##STR2## R.sup.1 's independently represent a hydrogen atom or an unsubstituted or substituted monovalent hydrocarbon group having from 1 to 6 carbon atoms, R.sup.2 's independently represent a hydrogen atom, an allyl group or a propenyl group, R.sup.3 represents an allyl group or a propenyl group, X represents a hydrogen atom or a halogen atom, and n is an integer of from 0 to 6. The derivative is useful for modifying curable resins or resin compositions to provide cured products which have a low water absorption, high strength and a high glass transition temperature. The derivative has good working properties and a good heat resistance.