Abstract: A sensitive resonance light scattering system for the quantitative determination of protein has been developed by using a new hydrophilic cyanine dye 1,1′-sulfonopropyl-3,3,3′,3′-tetramethylindolium-5,5′-disulfonic potassium. Effects of the concentration of dye, pH, ion-intensity and ethanol were studied and the optimum condition was gained. At the condition that pH was of 2.0, the resonance rayleigh scattering of cyanine enhanced remarkable with the addition of the protein. Good calibration curves of the protein were also obtained in the range of 0.20~15.0 μg/mL (BSA) and 0.50~10.0 μg/mL (HSA) with detection limits of 0.05 and 0.1 μg/mL respectively. Applied to artificial samples, the result was satisfied and the recovery of BSA and HAS was in the range of 94.5 %~104.0 %, and 95.6%~103.0% respectively.