Styrene-based resin composition containing as main components (A) 100 parts by weight of a styrene-based polymer having mainly a syndiotactic configuration and (B) 0.01 to 25 parts by weight of one or more nucleating agents selected from the group consisting of (1) metal salts of organic acids having an average particle diameter of not more than 50 .mu.m, (2) inorganic compounds having an average particle diameter of not more than 50 .mu.m, (3) organophosphorus compounds having an average particle diameter of not more than 50 .mu.m, and (4) metal salts of ionic hydrocarbon copolymer, and a process for producing styrene-based resin moldings by molding the above styrene-based resin composition at a temperature between room temperature and 200.degree. C. The resin composition of the present invention is suitable for injection molding and extrusion molding, and provides moldings which are of high crystallinity and are excellent in heat resistance, solvent resistance, chemical resistance and so forth.