A water-soluble porphyrin dimer (Por Dimer) containing eight positive charges, bridged by 4,4’-dicarboxy-2,2’-bipyridine, has been synthesized. With Meso-tetrakis(N-methyl-pyridium-4-yl)porphyrin (H2TMPyP) as the reference compound, the water-sol-uble porphyrin dimer was investigated for its inter-action with DNA by absorption, fluorescence, and circular dichroism (CD) spectroscopy. The apparent affinity binding constant (Kapp= 1.2×106) of Por Dimer binding to CT DNA was measured by a com-petition method with ethidium bromide (EB) (that of H2TMPyP was 6.9×106). The cleavage ability of Por Dimer to pBR322 plasmid DNA was studied by gel electrophoresis. The results suggest that the binding modes of Por Dimer were complex and involve both intercalation and outside binding.