DNA base modification as an important part of epigenetics includes DNA methylation and demethylation.It associated with cancer and neurodegeneration disease and many other diseases.The base modification of deoxycytidine()include 5-Methyldeoxycytidine(5mC),5-Hydroxymethyl-deoxycytidine(5hmC),5-formyldeoxycytidine(5fC),5-carboxyldeoxycytidine(5caC).Are there other modification of deoxycytidine existing in mamal DNA? To answer this question,we studied the metabolism of deoxycytidine in mESC.Method: 50 uM final concentration 15N3 labled deoxycytidine was exposured to mESC.After treated 24 hours,the whole genome DNA was extracted and then enzymatic hydrolyzed overnight[1].Finally,the sample was analyzed with UPLC-Q–TOF-MS after ultrafiltration.Results: There are significant MS signal of 15N2 labled deoxycytidine,15N2 labled Thymidine,15N2 labled 5mC,15N2 labled 5hmC,while deoxyadenosine and deoxyguanosine has not.The results indicate pyrimidine deoxynucleotide cycle is independent on purine cycle and and not find new modificaton on deoxycytidine.However,this method may be used to analyze the pyrimidine damage or modification.