Highly active catalysts for copolymerization have been prepared by the precipitation of MgCl2/TiCl4, complex with or without high surface area silica. Copolymerization of ethylene and 1-butene has been tested by using the prepared catalysts at various concentrations of 1-butene. The catalytic activities are 20-80 kg/g Ti h. The rate of copolymerization is strongly affected by the addition of l-butene. The decay rate of copolymerization is first order with respect to time. Analyses of copolymers with solvent extraction, DSC, IR, XRD, and NMR were performed. Ethylene reactivity ratio (k(11)) for TiCl4/MgCl2/THF catalyst is calculated to be about 26 by NMR spectrum.