PURPOSE: To obtain an optical disk substrate, by molding a resin consisting of an aromatic polyester and polyolefin and exposing the molded resin to radiation and improved heat resistance as well as transparency and small birefringence.

CONSTITUTION: An optical disk substrate produced by kneading (A) 70W95wt%, preferably 80W90wt% aromatic polyester with (B) 5W30wt%, preferably 10W20wt% low-molecular weight polyolefin having 1,000W10,000, preferably 2,000W5,000mol.wt. while heating, pelletizing the kneaded blend, molding the resultant pellets with an injection molding machine, heating the molded article, preferably in a vacuum at about the melting point of the polyolefin (B), exposing the article to radiation, e.g. at 90W110°C in an atmosphere under 2×10-4W5×10-6Torr vacuum degree, and irradiating the heated article with electron rays at 20W70Mrad using an electron accelerator.