PURPOSE: To provide the title compsn. having excellent thermal stability, etc. and improved properties with regard to dripping, by mixing a specified modified polyethylene terephthalate resin with a polyphenylene sulfide resin, a brominated polystyrene and a reinforcing filler.

CONSTITUTION: 100pts.wt. modified polyethylene terephthalate resin (A) having a heat crystallization temp. Tc(H) of not higher than 120°C is mixed with 0.5W5 pts.wt. polyphenylene sulfide resin (B), 3W30pts.wt. brominated polystyrene (C) (e.g., polytribromostyrene) and 5W200pts.wt. reinforcing filler (D) (e.g., glass fiber) to obtain the desired flame-retardant polyester resin compsn. The component A can be prepd. by introducing a modifier component such as polyethylene glycol, an adduct of ethylene oxide to bisphenol A or sodium polypropylene glycol diphthalate into a polyethylene terephthalate resin.