Highy active catalyst has been prepared by the reaction of MgCl2 with TiCl4 in THF, followed by precipitation in excess amount of n-hexane. Quantitative and qualitative analyses of the catalyst was performed by the combination of the methods such as spectrophotometry, atomic absorption, titration, and gas chromatrgraphy. This catalyst was tested for ethylene polymerization in the presence of AlEt2 with or without the addition of hydrogen. In this process the feasible reduction reaction could be proposed. The activity of the catalyst was lowered by the addition of hydrogen, while the initial activation of Ti sites was accelerated by the increase of the partial pressure of hydrogen. This phenomena were interpreted qualitatively on the basis of simple model. The change in the morphology of nascent polyethylene with the polymerization time was also studied. The speed of disintegration into subparticles and the pattern of polymer growth around the subparticles were considerably different with or without the addition of hydrogen. The change in the molecular weight with the polymerization time was also studied by GPC and viscometry method.