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 phosphoric acid ester morpholide compound or a mixture of phosphoric acid ester morpholide compounds. Another 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 polyphenylene ether, (C) about 0.1.about.100 parts by weight of a phenol resin derivative, and (D) about 0.1.about.50 parts by weight of a phosphoric acid ester morpholide compound or a mixture of phosphoric acid ester morpholide compounds. The thermoplastic resin composition containing polyphenylene ether resin may also contain up to 5.0 parts by weight of an anti-dripping agent based on 100 parts by weight of the base resin.