Abstract: A thermoplastic copolymer which comprises (a) 2.0 to 22.0% by weight of a methyl methacrylate unit, (b) 38.0 to 70.0% by weight of a styrene unit, and 20.0 to 45.0% by weight in total of both (c) a methacrylic acid unit and (d) an acid anhydride unit having a 6-membered ring structures. The acid anhydride unit has a 6-membered ring structure being in an amount of not less than 16.0% by weight per total amount of the units (c) and (d), and the copolymer has a reduced viscosity of 0.3 to 1.5 dl/g (measured in a 1% solution in dimethylformamide at 25.degree. C. which has well balanced heat distortion resisting properties, water absorption properties and mechanical strength and excellent heat stability in the fabrication thereof, and is useful for producing various parts such as automobile parts, electrical parts, industrial parts and miscellaneous goods.