The isomerization of methyl ester of cottonseed oil by the catalyst iodine was investigated. By the methods of GC-MS and UV. the constituents of isomerized products were analysed. UV absorbancy curves of measured samples at different concentra-tions were plotted, and the linear relationship of absorbancy and samples' concentration was detennined. Combined with GC-MS analysis results.the empirical formula for the quantitation of conjugated linoeic acid from isomerized products was obtained.