Monocyclopentadienyl compounds, CpMCl3 (M = Ti, Zr) supported on activated MgCl2 were used for the polymerizations of ethylene in the presence of methylaluminoxane (MAO) or a common alkylaluminium as a cocatalyst. By supporting CpMCl3 on MgCl2, the catalyst activity was increased drastically to show high activity similar to MgCl2-supported TiCl4 catalysts. The activity of the CpZrCl3/MgCl2 catalyst was higher than that of the CpTiCl3/MgCl2 one. Both catalysts gave polymers with high molecular weight (M,) and broad molecular weight distribution ((M) over bar(w)/(M) over bar(n)) in comparison with the corresponding soluble half-metallocene catalysts.