High activity ethylene polymerization catalysts have been prepared by the reaction of magnesium chloride with titanium tetrachloride in terahydrofuran, followed by precipitation with excess n-hexane. The catalysts are tested in ethylene polymerization in the presence of AlEt₃ in the slurry reactor. The yields of HDPE during 60min. polymerization at various polymerization conditions were obtained. The average molecular weight and molecular weight distribution have been examined by GPC and viscometry. Narrow molecular weight distributions of Q=3-11 were obtained. The effects of the concentration of monomer, alu-minum alkyls and hydrogen on molecular weight were investigated. The molecular weight of HDPE was found to be inversely proportional to the square root of the partial pressure of hydrogen. The morphologies of HDPE were also studied with SEM at various polymerization conditions.