A Ziegler-Natta catalyst was modified with a metallocene catalyst and its polymerization behavior was examined. In the modification of the TiCl4 catalyst supported on MgCl2 (MgCl2-Ti) with a rac-ethylenebis(indenyl)zirconium dichloride (rac-Et(Ind)(2)ZrCl2 EIZ) catalyst, the obtained catalyst showed relatively low activity but produced high isotactic polypropylene. These results suggest that the EIZ catalyst might block a non-isospecific site and modify a Ti-active site to form highly isospecific sites. To combine two catalysts in olefin polymerization by catalyst transitioning methods, the sequential addition of catalysts and a cc-catalyst was tried. It was found that an alkylaluminum like triethylaluminum (TEC) can act as a deactivation agent for a metallocene catalyst. In ethylene polymerization, catalyst transitioning was accomplished with the sequential addition of bis(cyclopentadienyl)zirconium dichloride (Cp2ZrCl2)/methylaluminoxane (MAO), TEE and a titanium tetrachloride/vanadium oxytrichloride(TCl4/VOCl3, Ti-V) catalyst. Using this method, it was possible to control the molecular weight distribution (MWD) of polyethylene in a bimodal pattern. In the presence of hydrogen, polyethylene with a very broad MWD was obtained due to a different hydrogen effect on the Cp2ZrCl2 and Ti-V catalyst. The obtained polyethylene with a broader MWD exhibited more apparent shear thinning.