The crystalline beta-form of ethylene/propylene random and block copolymers was obtained by crystallizing in the presence of a two-component beta-nucleator. The influence of converting the crystalline copolymers from the alpha-form to the beta-form an the drawing behaviour and impact strength was studied. The results show that the beta-phase copolymer has higher impact strength, slightly lower yield strength and a higher degree of stress whitening than the alpha-phase copolymer. A stretching-induced or impact-induced beta-alpha transformation in parallel with micro-crazing was also noticed.