An epoxy resin-based composition suitable for encapsulation of semiconductor devices is proposed. The resin composition is capable of giving a cured resin encapsulation highly resistant against crack formation by virtue of the unique ingredient therein, in addition to the thermally curable epoxy resin and an inorganic filler, which is a combination of an organopolysiloxane having a hydrocarbon group substituted by certain functional groups, e.g. glycidyl, amino, mercapto and the like, and a block copolymer composed of the segments of a polymeric aromatic moiety and segments of an organopolysiloxane moiety.