Objective:Preparated from seeds of Brassica alba, Sinapine was determinated by RP-HPLC. Methods: Seeds of Brassica alba were shattered, defatted by ether, extracted by alcohol, and crystallized by potassium thiocyanate, then ethanol and water were used to recrystallize for several times. The determination was performed by reversed-phase high performance liquid chromatography(RP-HPLC), using ZORBAX SB-Aq column 4.6 mm?250 mm, 5 ?m. The mobile phase consisted of acetonitrile-0.08 mol/L KH2P04 (20:80) at a flow rate of 1.0 ml/min, and the detection wavelength was 326 nm. Results: Pale yellow needle-like sinapine crystal was obtained, and the yield rate was 0.82%. The purity of sinapine thiocyanate crystal was over 95%. Conclusion: Preparation sinapine by this solvent extraction method was simple, high-purity, high yield, and could be used for industrial production.