A highly active Ziegler-Natta catalyst for polymerizing HDPE and LLDPE at low temperature and pressure may be prepared by reacting a magnesium compound and a halogen compound with an electron donor compound to dissolve, reacting the resulting solution with a titanium compound to obtain the homogeneous solution and then adding a small quantity of surfactant to the homogeneous solution. Ethylene polymers and copolymers were prepared by activating the resulting catalyst to obtain the granular polymer having density of from 0.92 to 0.98 g/cm.sup.3 and in which the particle size distribution was narrow.