Homo- and co-polymerization of ethylene were carried out in both gas and slurry phases over Mg(OEt)₂/THF/SiCl₄/TiCl₄-AlEt₃ catalysts in the range of temperature 20-70℃ and pressure 2-10 psig. In gas phase polymerization, maximum activity was measured at the Al/Ti mole ratio of 377, and reaction rate dependence on AlEt₃ concentration could be explained with the Langmuir-Hinshelwood adsorption model. Even though maximum activities were obtained at the same temperature, 60℃ in both gas and slurry phases, overall activation energy was higher for the slurry phase(13 ㎉/㏖) than for the gas phase(4.7 ㎉/㏖) polymerization. The molecular weight behavior has been examined by measuring intrinsic viscosity. The molecular weight was increased as the ethylene pressure increased, and as the temperature and the concentration of AlEt₃ and hydrogen decreased. Using two different comonomers(propylene and butene-1), the copolymerization of ethylene was carried out. The intrinsic viscosity and the melt index were decreased as comonomer concentration increased, and the maximum activity was observed when the concentration of comonomer is about 20 ㏖%.