The silicone composition for sealing a semiconductor, comprises: (A) 100 parts of a polyorganosiloxane having one or more alkenyl groups, obtained by reacting (a1) 60 to 99 parts of an organosiloxane containing at least a trifunctional siloxane unit not taking part in a hydrosilylation reaction, with (a2) 40 to 1 parts of an organosiloxane containing a bifunctional siloxane unit having an alkenyl group and/or a monofunctional siloxane unit having an alkenyl group; (B) an amount of a polyorganohydrogensiloxane having two or more hydrogen atoms having a viscosity at 25° C. of 1 to 1000 mPa·s so that an amount of the hydrogen atoms is 0.5 to 3.0 mol per mol of the alkenyl groups; and (C) a platinum-based catalysts, wherein a decrease in storage modulus of a cured product thereof from 25° C. to 50° C. is 40% or more.Specifically the polyorganosiloxane (A) is prepared by block or graft polymerization and equilibration reaction of three specific, different, organosiloxanes including a branched siloxane, a linear siloxane and a cyclic siloxane.