Two imidazolinyl quaternary ammonium salt M1 and M2 were synthesised using naphthenic acid, diethylene triamine (or triethylene tetramine), and diethyl sulfate. The interaction between fsDNA respectively with M1 and M2, enhanced the intensities of resonance light scattering (RLS) significantly in Britton Robinson buffer solution of the pH 2.36 for M1 and 3.78 for M2. Also, it was studied the effect factors on the reaction between imidazolinyl quaternary ammonium salt and fsDNA. Under the optimum conditions, the enhanced intensities of RLS had good linear relationship with the concentration of fsDNA. Thus, a new RLS method for determination of fsDNA was established. The method has the properties of high sensitivity, simple operation, good accuracy and the wide range of linear. It can be used to the determination of fsDNA containin the synthesized samples with the result of satisfying.