A polyorganosiloxane-containing graft copolymer composition which comprises: a polyorganosiloxane-containing graft copolymer (A) obtained by polymerizing 5 to 60 parts by weight of a vinyl monomer (a-2) in the presence of 40 to 95 parts by weight of polyorganosiloxane particles (a-1) (the sum of (a-1) and (a-2) is 100 parts by weight); and an antioxidant (B). The composition is useful as a flame retardant for thermoplastic resins.
