To modify a polymer blend composed mainly of a polycarbonate resin and a polystyrene resin to a thermoplastic resin compsn. having fluidity and impact strength comparable to a polycarbonate resin blended with an ABS resin and to provide a harmless, flame retarded thermoplastic resin compsn. based on the modified compsn.

This thermoplastic resin compsn. comprises: 100 pts.wt. resin blend comprising 30 to 95wt.% polycarbonate resin (a) and 5 to 70wt.% polystyrene resin (b) and 0.1 to 10 pts.wt. ethylene/carbon monoxide/(meth)acrylic alkyl ester copolymer incorporated into the blend. Further, 1 to 40 pts.wt. organophosphorus compd. (d) and 0.05 to 5 pts.wt. fluoroethylene polymer (e) are added.