PURPOSE: A mixture of a metal salt of higher fatty acid and glycerol monostearate is incorporated to a polystyrene resin to produce said composition having good releasability from mold at high temperatures.

CONSTITUTION: (A) 100 parts by wt. of polystyrene resin and (B) 0.07W0.4 parts of a mixture consisting of a higher fatty acid as lauric acid and/or a metal salt of higher fatty acid as barium stearate, and glycerol monostearate in an amount of 0.2 W 2.5 times the amount of the former (A) are kneaded in an extruder. The resulting composition is suitably used in injection blow molding requiring mold release at high temperatures.