The present invention relates to a halogen free flame retardant thermoplastic styrene resin composition. More particularly, the present invention relates to a flame retardant resin composition, comprising (A) a graft copolymer consisting of rubber modified styrenic resin, (B) a polycarbonate resin, (C) a phenol novolak resin, (D) a copolymer consisting of styrene, and (E) an aromatic phosphate. The resin composition of the present invention has a superior flame retardancy using a halogen free flame retardant and a little amount of polycarbonate resin.