Al-Ti oxychloride was prepared by reacting AlCl3and TiCl4with the equimolar amount of water in ethyl ether, followed by thermal treatment of the reaction product. Polymerization of ethylene with the catalyst consisting of AI-Ti oxychloride and Al(C2H5)3or (C2H5), AlCl was studied. With respect to the catalyst activity, Al/Ti oxychloride having a ratio of 1/1 was found most desirable, and the chlorine content in the oxychloride was of importance and significance. Hydrogen was used for controlling the molecular weight of the polymer. Two peaks of high catalyst activity were observed, one at about 70°C (slurry region) and the other at about 160°C (solution region). Effects of the conditions of polymerization with Al-Ti oxychloride-(C2,H5)2AlCl catalyst on the structure of the polyethylene, which was found to be of high density and high stiffness, are discussed. The amount of double bonds in the polyethylene was very small, but they were mostly of the vinyl group type.