Abstract: The geometries, electronic structures and spectroscopic properties for a series of osmium(II) complexes [Os(II)(CO)3(tfa)(L)](tfa=trifluoroacetate; L=O^O (1), O^N (2),N^N (3), O^O=hexafluoroacetylacetonate, O^N=quinolinolate, N^N=3-(trifluoromethyl)-5-(2-pyridyl) pyrazole) were investigated theoretically. The Becke’s three parameter functional, the Lee-Yang-Parr (B3LYP) functional and single-excitation configuration interaction (CIS) methods were used to optimize the ground and excited states for complexes 1-3. The time-dependent density functional theory (TD-DFT) at B3LYP level with the polarized continuum model (PCM) was employed to obtain their absorption and phosphorescent emission spectra in CH2Cl2 based on their optimized ground and excited states geometries. The results revealed that the optimized structural parameters agreed well with the corresponding experimental results. There were slight structural differences between ground and excited states, and this was in agreement with the small Stokes-shift observed in the experiments. The lowest-lying absorptions were at 342, 431and 329 nm, and phosphorescent emissions were at 521, 638 and 488 nm for complexes 1-3, respectively. The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of 1-3 mainly manifest as characteristic π and π* orbitals of the L ligands. The lowest-lying absorptions were thus assigned to π-π* transitions with some metal to ligand charge transfer (MLCT) and ligand to ligand charge transfer (LLCT) contributions. High energy absorptions also had intraligand charge transfer (ILCT) and LLCT characteristics. The phosphorescence emissions of these complexes have similar transition properties to their absorptions.