The present invention relates to a flame retardant thermoplastic resin composition that contains a phenol resin derivative having good char formability, regardless of the base resin. A flame retardant thermoplastic resin composition according to the present invention comprises (A) 100 parts by weight of a thermoplastic resin as a base resin, (B) about 0.1.about.100 parts by weight of a phenol resin derivative, and (C) about 0.1.about.50 parts by weight of a phosphorous compound or a mixture of phosphorous compounds.