Abstract: Objective: To establish a method to determine sunset yellow content of orange juice. Methods: Poly ( diallyldimethylammonium chloride) ( PDDA) capped silver nanoparticles ( Ag) was prepared via a simple chemical reduction using PDDA as a stabilizer and Na BH₄ as a reducing agent for Ag NO₃. The interaction between PDDA-Ag and sunset yellow was studied by UV-vis spectrophotometry.Results: Under the optimal conditions, a good linear relationship of ΔA with respect to concentration of sunset yellow cross the range of 0.01 × 10^-6~ 9 × 10^-6mol/L was achieved. The proposed method was successfully applied for the determination of sunset yellow in orange juice with recoveries 94% ~ 108%. Conclusions: This method could provide experimental data and method for detection of sunset yellow in the sample.