In a copolymerization process of propylene, a mixture of unreacted propylene and ethylene is recovered. It is then fed to a distillation system which includes first and second distillation columns. The distillation system is operated in such a way that the temperature and pressure of the top of the second distillation column remain constant and an ethylene-propylene mixture can be recovered at a constant ethylene/propylene ratio from the top of the second distillation column. By using the thus-recovered ethylene-propylene mixture, the copolymerization reaction can be carried out under excellent control so that the ethylene-propylene copolymer can be obtained with uniform quality and properties.