N, N'-bis(2-pyridinecarboxamide)-1,2-ethane(H2L1), N, N'-bis(2-pyridinecarboxamide)-1, 2-beneze(H2L2) and N-phenylpyridine-2-carboxamide(HL3) were synthesized, and characterized by elemental analysis, IR and HNMR spectra. UV-visible (UV-Vis) spectra, fluorescence spectra and SERS spectra to study the interaction of the three ligands with calf thymus DNA. UV-visible (UV-Vis) spectra show that with the incremental addition of DNA, the bands of H2L1, H2L2 and HL3 all show Hypochromism. Meanwhile fluorescence spectra show that the addition of the three ligands to DNA pretreated with EB causes an appreciable reduction in fluorescence intensity, indicating that the ligands compete with ethidium bromide in binding to DNA, and free ethidium bromide increases. The addition of DNA causes the SERS signals of the ligands to weakened and some bands to disappeared. Based on the above experimental results, we conclude that the three ligands bind to DNA mainly through the intercalation mode. The binding constant of the three compouds Kb was calculated, 1.20 x 10(4) for H2L1, 1.33 x 10(4) for H2L2 and is 1.52 x 10(4) for HL3. Kr was also calculated to be 0.67, 1.52 and 1.73 for H2L1, H2L2 and HL3, respectively. The value indicates that the binding of HL3 to DNA is stronger than that of H2L1 and H2L2, as HI3 has proper planar structure, smaller molecular volume and less steric hindrance. The three ligands can all induce the cleavage of plasmid pBR322 DNA. An increase in H2L1, H2L2 and HL3 concentrations causes more transformation of plasmid DNA from closed circular conformations to nicked conformations. But linear conformations have not been observed. The cleavage of plasmid pBR322 DNA caused by the three ligands is not selective.