Novel ethylene-propylene random copolymers with isotactic sequence of propylene were synthesized with a MgCl2-supported Cr(acac)(3) catalyst. The propylene content was controlled by the monomer concentration. The crystallinity of these copolymers showed a minimum around 50 mol% content of propylene, or ethylene. According to differential scanning calorimetry and wide-angle X-ray diffraction, crystals of polyethylene (PE) and polypropylene (PP) do not coexist, and ethylene-rich copolymers exhibit the orthorhombic form of PE and propylene-rich copolymers exhibit the monoclinic form of PP. It was found that the mechanical properties of these changed drastically between 70 and 53 mol% of propylene content.