The solution of $[Ru(phen)_2(dppz)]^{2+}$ and SDS has high Resonance Light Scattering (RLS) signals due to $[Ru(phen)_2(dppz)]^{2+}$ assemble on the surface of the SDS micelle. Because of the high affinity ($KBgeq10^6;L;mol^{-1}$) between $[Ru(phen)_2(dppz)]^{2+}$ and DNA, the adding of DNA in the solution of $[Ru(phen)_2(dppz)]^{2+}$-SDS makes the dissociation of $[Ru(phen)_2(dppz)]^{2+}$-SDS, and results in decreasing of the RLS signals and increasing of the absorbance. Based on this, a novel method is proposed for DNA assay. Under optimum condition, good linear relationship was obtained within the concentration range of 0.018-1.26 $mu g;mL^{-1}$, the linear equation is $I_{RLS}$ = 504.8-348.8 c (c: $mu g;mL^{-1}$) and the correlation coefficient (r) is 0.9992. The detect limit for calf thymus DNA is 8.6 ng $mL^{-1}$. The proposed method was successful applied to determine the extracted colibacillus plasmid DNA.