To improve the toughness of silicone-modified epoxy resin, aramid-silicone block copolymer was used as a compatibilizer. Fine silicone phases could be uniformly and stably dispersed in epoxy matrix by the addition of the block copolymer grafted with epoxy oligomers. The architecture of the block copolymer, such as the silicone content of the block copolymer and the molecular weight of epoxy and silicone segments, strongly affected the effectiveness of a compatibilizer. The block copolymer that acts as a good compatibilizer was mainly concentrated in an interfacial area around the silicone phase. The thickness of the interphase depended on the architecture of the block copolymer and decreased with an increase in the chain length of the epoxy segment. The values of fracture toughness in the silicone-modified system increased with a decrease in the diameter of the silicone phases. Their maximum value increased to about 2.0 times that of the unmodified system.