The electronic structures and spectral properties of three Re(I) complexes [Re(CO)(3)XL] (X = Br, Cl; L = 1-(4-5 '-phenyl-1.3,4-oxadiazolylbenzyl)-2-pyridinylbenzoimidazole (1), 1-(4-carbazolylbutyl)-2-pyridinylbenzoimidazole (2), and 2-(1-ethyl benzimidazol-2-yl)pyridi ne (3)) were investigated theoretically. The ground and the lowest lying triplet excited states were full optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ levels, respectively. TDDFT/PCM calculations have been employed to predict the absorption and emission spectra starting from the ground and excited state geometries, respectively. The lowest lying absorptions were calculated to be at 481, 493, and 486 nm for 1-3, respectively, and all have the transition configuration of HOMO -> LUMO. The lowest lying transitions can be assigned as metal/ligand-to-ligand charge transfer (MLCT/LLCT) character for 1, ligand-to-ligand charge transfer (LLCT) character for 2, and mixed MLCT/LLCT and intraligand pi-pi* charge transfer (ILCT) character for 3. The emission of 1 at 551 nm has the (MLCT)-M-3/(LLCT)-L-3 character, 2 has the (MLCT)-M-3/(LLCT)-L-3 character at 675 nm, and the 651 nm transition of 3 has the character of (MLCT)-M-3/(LLCT)-L-3/(ILCT)-I-3. Ionization potentials (IP) and electron affinities (EA) calculations show that the comparable EA and smaller IP values and the relativly balanceable charges transfer abilitie of 2 with respect to I and 3 result in the higher efficiency of OLEDs. The calculated results show that the absorption and emission transition character and device's efficiency can be changed by altering the ancillary ligands.