Provided are a curable composition and its use. The curable composition may provide a cured product having excellent processability and workability, no whitening and surface stickiness, and an excellent adhesive property. Since the curable composition has excellent thermal resistance, gas barrier-ability, and crack resistance, even when a semiconductor device to which the composition is applied is used at a high temperature for a long time, performance of the device may be stably maintained. Specifically the curable composition is a blend of three different polyorganosiloxanes having aliphatic unsaturated groups and specific Ar/Si ratios and a polyorganosiloxane having SiH groups.