Abstract: ESR, ¹H-NMR and IR were combined so as to give information concerning the mechanism of butadiene polymerization in the presence of catalyst system Fe(phen)Cl₃-(i-C₄H₉)₃Al. It was shown by the ESR spectra that the iron in active species is reduced into Fe(Ⅱ) by triisobatyl aluminium during the polymerization and the signal of free radical could be observed,so a reduction mechanism is proposed. Two systems, Fe(phen)Cl₃-(i-C₄H₉)₃Al and Fe(phen)Cl₃-(i-C-4H₉)₃Al-butadiene, have diffferent spectra in ESR. The former gives a very wide signal(△H_pp=900G), but the latter shows a more sharp one (△H_pp=50G).The overall polymerization mechanism has been proposed, which is supported by ¹H-NMR and IR spectra measurements of the system.