A metallocene catalyst or metallocene/Ziegler-Natta hybrid catalyst supported on MgCl₂ was studied for application to the commercial slurry phase process and to the control of MWD of the polymer produced. The MgCl₂ support was prepared by the recrystallization mehtod with different solvents: alcohols and H₂O. The recrystallized MgCl₂ support was pretreated with trisobutylaluminum (TiBAL) or trimethylaluminum (TMA), methylaluminoxane (MAO), to give different supporting environments. Metallocene/Ziegler-Natta hybrid catalysts on MgCl₂ were prepared by first supporting the metallocene catalyst, followed by the Ziegler-Natta catalyst after pretreatment with an alkylaluminum compound. The type of solvent plays a role in providing a suitable environment in the support for impregnating catalysts. The hybrid catalyst, whose support was recrystallized in n-propanol (n-PrOH) and pretreated with TiBAL, showed the highest molar ratio of Zr/Ti=1/2.71. But the hybrid catalyst, whose support was recrystallized in H₂O and pretreated with MAO, showed the best hybrid effect showing a variety of bimodal peaks in proportion to the ratio of MAO/TEA in GPC analysis. This effect might be due to steric factor and polarity of solvent, as well as the pretreatment material. It is concluded that the MWD of polyethylene produced by metallocene/Ziegler-Natta hybrid catalyst that is supported on MgCl_2, can be controlled by regulating the amounts of MAO and TEA added as cocatalysts and by the use of different solvents and pretreatment materials.