The charge carrier transporting ability in the polymorphism of tris(8-hydroxyquinolinato)aluminurn(III) (Alq(3)) has been studied using density functional theory (DFT) and Marcus charge transport theory. alpha- and beta-Alq(3) composed of mer-Alq(3) molecules have stronger electron-transporting property (n-type materials) compared with their hole-transporting ability. In contrast, gamma- and delta-Alq(3) formed by fac-Alq(3) molecules possess stronger hole-transporting character than their electron-transporting ability. The detailed theoretical calculations indicate the reason lies in the differences of HOMO and LUMO distribution states of the two kinds of isomers, and the different molecular packing modes of charge-transporting pathways for different phases.