Abstract: Determination and optical induced heterogeneous phenomenon of Sudan was studied using a Shimadzu LC-20 AB liquid chromatograph equipped with a SPD- M20 A diode array detector. Chromatographic separations were performed using a CNW Athena C18-WP( 4.6 mm × 150 mm,5 μm) column,the column temperature was 30 ℃.The mobile phase consisted of acetonitrile- water( 88∶ 12 by volume) and was delivered at a flow rate of 1.0 m L / min with equivalent elution. The detection wave length was 500 nm. The results showed that Sudan Ⅰ- Ⅳ were linear within a range of 0.10 ~ 20.00 mg / L,and the correlation coefficient was 1.0000. Sudan Ⅰ and Ⅱ could not occur optical induced heterogeneous phenomenon,but Sudan Ⅲ and Ⅳ could.Optical induced heterogeneous of Sudan was a reversible change,and it could be reversed when there was no illumination condition. Sudan standard solution was more easily to occur optical induced heterogeneous than solid Sudan standard.Temperature had little effect on optical induced heterogeneous of Sudan in the range of 0 ~60 ℃. Illumination had a significant effect on optical induced heterogeneous phenomenon of Sudan,the degree was brown vial > sunlight = 7 W energy saving light.Concentration of Sudan also had a significant impact on its optical induced heterogeneous phenomenon.The higher the concentration,the longer the time that heterogeneous tended to stable needed.Within the range of 0.50~20.00 mg / L,optical induced heterogeneous rate of Sudan III was 5.324% ~ 5.787% in the state of heterogeneous balance,and Sudan Ⅳ was 5.121% ~ 5.507% under that condition. The study had important reference value in research and application of Sudan detection methods.