Three novel rare earth complexes, [Ln2(DCA)2(phen)2](NO3)2·6H2O (Ln(Ⅲ)=Sm(Ⅲ)(1), Er(Ⅲ)(2), Yb(Ⅲ)(3); DCA2-=demethylcantharate, 7-oxabicyclo[2.2.1] heptane-2,3-dicarboxylate, C8H8O52-; phen=1,10-phenanthroline, C12H8N2) were synthesized. The structures were characterized by elemental analysis, molar conductance, IR and TGA. The results suggested that the structural features of the complexes were: in each DCA2-, one carboxylate group, as bidentate bridging group, connected two rare earth ions; the other carboxylate group, as bidentate chelate group took part in the coordination with rare earth ion. And cyclic ether oxygen of DCA2- and nitrogen atoms of phen took part in the coordination. The probable coordination number was seven. The interaction of the complexes with DNA was studied by UV-spectra, fluorescence spectra and viscosity measurements. Following increasing the concentration of DNA, the UV absorption bands nearby 265 mn of the three complexes appeared hypochromism and red-shift phenomena. And the values of binding constants Kb were 1.89× 105 L/mol (1), 3.54× 104 L/mol (2) and 3.83× 104 L/mol (3). The complexes could quench the fluorescence of EB-DNA system, and the values of equilibrium constants Ksq were 1.72(1), 0.56(2) and 1.09(3). The relative viscosity of DNA steadily decreased with increasing theconcentration of complexes. So, we could infer that the complexes may partially insert into DNA. The study of agarose gel electrophoresis showed that the complexes could cleave plasmid DNA, and the process of the reaction was through unclassical redox mechanism.