Abstract: Methyl esterification of fatty acids is required prior to gas chromatography-combustionisotope ratio mass spectrometry (GC-C-IRMS) analysis, but each fatty acid methyl ester(FAME) contains a methyl group from derivatization that is not present in the original fatty acid. So it is necessary to correct the δ¹³C value of each FAME for the δ¹³C value of the extra methyl carbon. In order to obtain the accurate δ¹³C value of the methanol used to methylate the fatty acids, and then improve the accuracy of the corrected δ¹³C values of fatty acids , this paper analyse the δ¹³C values of the methanol as determined by three different methods: 1) The δ¹³C values of sodium methoxide solid from chemical reaction between the methanol and sodium are determined by off-line method; 2) The δ¹³C values of methanol crystal as prepared by a special method that liquid nitrogen cools down quartz tube are determined by off-line method; 3) The δ¹³C values of the methanol are determined by GC-C-IRMS with headspace sampling. Besides, the δ¹³C values of the fatty acids and FAMEs are also determined by off-line method, and the δ¹³C values of the methanol as determined by three different methods correct the δ¹³C values of each FAME respectively, and then this paper compare the calculated δ¹³C values of the fatty acids with that of the fatty acids as determined by off-line method. The results show that the average δ¹³C value of the methanol crystals as determined by off-line method is -50.17‰, and the standard deviation is 0.086‰ （n=5）. Reproducibility of these values is the best, and the calculated δ¹³C values that the δ¹³C values of the FAMEs are corrected by the δ¹³C values of the methanol are the most similar with the δ¹³C values of the fatty acids as determined by off-line method . So it is a reliable method that methanol crystals are determined by off-line method, and then its accuracy is improved that the δ¹³C value of the methanol correct carbon isotope measured values of FAMEs.