The structures and related properties of the complex [Ru(phen)(2)(6-OH-dppz)](2+) (phen = 1,10-phenanthroline; dppz = dipyrido [3,2-a:2',3'-c]phenazine) in the ground state (S-0), the first singlet excited state (S-1), and the first triplet excited state (T-1) have been studied using density functional theory (DFT), time-dependent (TD) DFT, Hartree-Fock (HF), and configuration interaction singles (CIS) methods. Three electronic absorption-spectral bands ((MLCT)-M-1, (LL)-L-1, and (LL)-L-1) lying in the range of 250-550 nm in vacuo and in aqueous solution were theoretically calculated, simulated, and assigned with TDDFT method. In particular, the theoretical results show the following: (1) The positive charges of central Ru atom in the excited states (S-1 and T-1) are greatly increased relative to those in the ground state (S-0), and thus the Ru atom in the excited states can be regarded as Ru(III). (2) The positive charges on the main ligand (6-OH-dppz) in the excited states are considerably reduced, and thus the interaction between the main ligand (intercalative ligand) and DNA base pairs is considerably weakened. (3) The geometric structures in excited states are also distorted, resulting in obvious increase in the coordination bond length. It is advantageous to the complex forming a high oxidizing center (i.e., Ru(III) ion). On the basis of these results, a theoretical explanation on photoinduced oxidation reduction mechanism of DNA photocleavage by [Ru(phen)(2)(6-OH-dppz)](2+) has been presented.