Abstract: The effect of halogen atom in the (CF₃COO)₂NdX·EtOH-R₃Al (X:Cl, Br,I;R:Et,i-Bu) catalyst system on the polymerization of butadiene or isoprene has been studied. The results indicate that the catalyst containing halogen atom with higher eletro-negativity is more active. The intrinsic viscosities of polybutadiene and polyisoprene catalyzed with (CF₃COO)₂NdX·EtOH-Et₃Al system were found in the order of Br>Cl>I while that obtained with (CF₃ COO)₂NdX·EtOH-i-Bu₃Al system Cl>Br>I. The catalysts with different halogen lead to the microstructure change of the polymer obtained, but the active species of the catalyst system have little effect on the cis-contents of polymers.