Studies on the electronic structures and trend in DNA-binding affinities of a series of Ru(II) complexes [Ru(bpy)(2)(p-R-pip)](2+) (bpy = 2,2-bipyridine; pip = 2-phenylimidazo[4,5- f] [1,10]-phenanthroline; R = -OH, -CH3, -H, -NO2) 1-4 have been carried out, using the density functional theory (DFT) at the B3LYP/ LanL2DZ level. The electronic absorption spectra of these complexes were also investigated using time-dependent DFT (TDDFT) at the B3LYP//LanL2DZ/6-31G level. The computational results show that the substituents on the parent ligand ( pip) have a significant effect on the electronic structures of the complexes, in particular, on the energies of the lowest unoccupied molecular orbital (LUMO) and near some unoccupied molecular orbitals (LUMO + x, x = 1-4). With the increase in electron-withdrawing ability of the substituent in this series, the LUMO + x (x = 0-4) energies of the complexes are substantially reduced in order, for example, epsilon(LUMO)(1) approximate to epsilon(LUMO)(2) > epsilon(LUMO)(3) > epsilon(LUMO)(4), whereas the pi-component populations of the LUMO+x (x = 0-4) are not substantially different. Combining the consideration of the bigger steric hindrance of complex 2, the trend in DNA-binding affinities (K-b) of the complexes, that is, K-b(2) < K-b(1) < K-b(3) < K-b(4) can be reasonably explained. In addition, the experimental singlet metal-to-ligand charge transfer ((MLCT)-M-1) spectra of these complexes can be well simulated and discussed by the TDDFT calculations.