DNA methylation level in Apostichopus japonicas tissue was determined by high performance liquid chromatography(HPLC) with ultraviolet detection,and the variations of DNA methylation level in Apostichopus japonicas tissue induced by quinoxaline were studied.The chromatographic conditions were as follows:column:ZORBAX SB-Aq(4.6 mm×250 mm,5 μm),column temperature:30 ℃,detection wavelength:280 nm; flow rate:1.0 mL/min,mobile phase：methanol-7 mmol/L ammonium acetate(7∶93).Under the optimal conditions,the calibration curves were linear in the concentration range of 0.05-1.0 mg/L for 5-methyl deoxidization cytidine(5-dmC) and deoxycytidine(dC),with correlation coefficients of 0.999 9.The detection limits were both 0.05 mg/L.The recoveries were in the range of 90.4%-100.6% and the relative standard deviations were below 4% at spike levels of 0.05,0.25,1.0 mg/L.Apostichopus japonicas tissues DNA were extracted using CTAB and hydrolyzed with enzyme.The 5-dmC and dC were determined using HPLC and the results showed that the DNA methylation level of the quinoxaline groups were lower than that of the control groups.Quinoxaline affected the normal gene expression by changing DNA methylation level,which might be a cause of genetic toxicity.The method could be used for the detection of dC and 5-dmC,thus enabling the evaluation of global DNA methylation.