A series of ruthenium(II) complexes with electron-donor or electron-acceptor groups in intercalative ligands, [Ru(phen) 2+ (3) and [Ru(phen) 2 (o-NP)] 2+ (4), have been synthesized and characterized by elementary analysis, ES-MS, 1 H NMR, electronic absorption and emission spectra. The binding properties of these complexes to CT-DNA have been investigated by spectroscopy and viscosity experiments. The results showed that these complexes bind to DNA in intercalation mode and their intrinsic binding constants (K b ) are 1.1, 0.35, 0.53 and 1.7 Â 10 5 M )1 , respectively. The subtle but detectable differences occurred in the DNA-binding properties of these complexes are mainly ascribed to the electronwithdrawing abilities of substituents (-OCH 3 < -CH 3 < -Cl < -NO 2 ) on the intercalative ligands as well as the intramolecular H-bond (for substituent -OCH 3 ) which increase the planarity area of the intercalative ligand to some extent. The density functional theory (DFT) calculations were also performed and used to further discuss the trend in the DNA-binding affinities of these complexes.