The structures and spectroscopic properties of a series of (C^N)2Ir(N^N-X) [C^N=2-phenylpyridine, N^N = 2-pyridyl-1,3,4-oxadiazole, X=chlorobenzene (1), X=phenyl(2), X=phenylyne (3), X=pyridine (4)] complexes were investigated by the DFT and TDDFT methods. We first optimized geometrical structures in ground state of complexes 1-4, at the B3LYP/LanL2DZ levels, and the excited state structures by selected UB3LYP/LanL2DZ functionals. The calculated bond lengths of Ir-N(1) and Ir-N(3) in the ground state agree well with the corresponding experimental results. At the TD-DFT and PCM levels, 1-4 give rise to lower energy absorptions at 454(1), 446(2), 453(3), 461(4) nm and corresponding phosphorescent emissions at 649(1), 650(2), 642 (3), and 638(4) nm, respectively. The HOMO-1 of 1-4 are mainly localized on the phenylpyridine(C^N) ligands, while the LUMOs are the *(N^N) type orbital. Therefore, the transitions of 1-4 were assigned to the π-π* transition with ligand-to-ligand charge transfers(LLCT) characters. It is shown that the absorptions and emissions are significantly dominated by the bidentate ligand itself, but are minor impact by the X-substituent group.