Disclosed herein is a flame retardant thermoplastic resin composition having excellent weatherability comprising (A) about 100 parts by weight of a rubber modified aromatic vinyl resin comprising (a1) about 15 to about 100% by weight of a grafted copolymer resin prepared by graft-polymerizing about 5 to 65% by weight of a rubbery polymer, about 10 to 95% by weight of an aromatic vinyl monomer and about 0 to 40% by weight of a monomer copolymerizable with said aromatic vinyl monomer; and (a2) about 0 to 85% by weight of a copolymer resin prepared by polymerizing about 40 to 90% by weight of an aromatic vinyl monomer and about 10 to 60% by weight of a monomer copolymerizable with said aromatic vinyl monomer; (B) about 0.1 to about 10 parts by weight of a ring-shaped alkyl phosphonate ester compound; and (C) about 0.05 to about 3 parts by weight of a HALS compound.