This paper reports a DFT study on trans-1,4-specific polymerization of isoprene catalyzed by the cationic heterobimetallic half-sandwich complex [(C5Me5)La(AlMe4)](+). The possible structures of the active species, viz., [(C5Me5)La(mu(2)-Me)(3)AlMe](+) (A), [(C5Me3)La(mu(2)-Me)(2)AlMe2](+) (B), and [(C5Me5)La(Me)(mu(2)-Me)AlMe2](+) (C), have been investigated. On the basis of the chain initiation and the structure transformations among these three species, C has been proposed to be the true active species smoothly producing trans-1,4-polyisoprene observed experimentally. Both La/Al bimetal-cooperating monomer insertion and La-center-based insertion pathways have been calculated, and the latter is found to be more favorable, where the AlMe3 moiety serves as a ligand coordinating to the La center via a methyl group. In contrast to this, in the Y analogous system, the AlMe3 ligand is proposed to leave away from the Y center during the chain propagation and the cis-1,4-selectivity is preferred, showing a consistence with experimental results. Such a situation could be ascribed to the smaller ionic radius of Y and thermodynamically favorable dissociation of AlMe3 from Y center in comparison with the La system. These results suggest that such an alkylaluminum compound plays a crucial role in the regulation of selectivity in the polymerization system investigated.