To obtain a polyester resin composition which is excellent not only in transparency and molding workability, but also in mechanical characteristics and heat deformation resistance.

This polyester resin composition comprises 100 pts.wt. polymer resin composition consisting of 5-95wt.% polyethylene terephthalate or a polyester mostly containing it and 95-5wt.% polyarylate, 0.01-1 pts.wt. alkali metal salt or alkaline earth metal salt of a 2-4C saturated aliphatic monocarboxylic acid, and 0.01-1 pts.wt. phosphor compd. It is melt and kneaded with a mono- or di- axial extruder having a screw with a length to diameter ratio of 20/1-30/1 under a temp of not less than 260°C and with a discharge capacity of 4-10kg/h.