Thermally pretreated catalysts were prepared by heating MgCl2/THF/ TiCl4 (TT-0) 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). Ethylene-l-hexene copolymers were prepared with these catalysts. The TT-1 catalyst produced more blocky and higher l-hexene content polymer than TT-0, 2, 3, and 4. Temperature rising elution fractionation (TREF) analysis was used to investigate the chemical composition distribution of the ethylene-l-hexene copolymer, exhibiting. bimodal distribution for TT-0 and trimodal for TT-1, 2, 3, and 4. A portion of higher hexene content of the copolymer markedly increased when the copolymerization was performed with TT-1, indicating that copolymerization active sites were newly generated. Portion of homopolyethylene increased drastically when the copolymerization was performed with TT-4, indicating that ethylene homopolymerization active sites were increased. Gel permeation chromatography (GPC) also revealed that three kinds of active sites existed on the catalyst. C-13-NMR spectrum of each fraction after TREF analysis suggested that the isospecific active site could polymerize I-hexene well, resulting in random and alternating copolymers. A scheme for generation of the active site and change of its nature during thermal treatment of bimetallic complex catalyst is proposed.