MgCl2/THF/TiCl4 (TT-0) were thermally pretreated at 80 degrees C for 5 min (TT-1) and 60 min (TT-2), and at 108 degrees C for 5 min (TT-3) and 60 min (TT-4). These thermally pretreated catalysts showed comonomer enhancement effects in the ethylene-1-hexene copolymerization, while TT-0 catalyst did not. Comonomer enhancement effect of thermally pretreated catalysts could come from the generation of new active sites and change of its nature after heat treatment. l-Hexene content in copolymer obtained with TT-1 was higher than those of TT-4 and TT-0. The morphology of homopolyethylene (PE) obtained with TT-1, 2, 3, and 4 was more regular and homogeneous than that of TT-0. This result could be due to the generation of active sites and change of its nature after thermal treatment of bimetallic catalyst.