The ground,ionic and singly excited structure of the hole-blocking material,3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-l,2,4-triazole (TAZ),has been optimized respectively by density functional theory (DFT) and single-excitation configuration interaction (CIS) methods.Calculated emission energy of TAZ is shown to be consistent with the experimental data.By analyzing the partial density states (PDOS) of these ligands contributing to the total density of states (TDOS),it is concluded that TAZ still favor electron carrier,but cannot do so under positive charged state.