A sulfuric acid catalyzed esterification method was chosen for the determination of undecylenic acid(UA) and zinc undecylenate(ZnUA) in cosmetics.Effects of different operation parameters such as methyl esterification reagent，sulfuric acid concentration，water content in reaction system，reaction temperature and time were investigated.The result indicated that water content in reaction system had a great impact on esterification reaction.Under the optimal conditions，heated in 65 ℃water bath for an hour，UA and ZnUA were esterified with 5% sulfuric acid-methanol using n-hexane as extraction solvent.Methyl undecenoate was analyzed by GC with hydrogen ion flame detector(FID) and quantified by the external standard method.The yield of methyl undecenoate was calculated to be above 95%.The method had a good linearity in the range of 0.05-1.0 mg with correlation coefficient more than 0.999. The limit of detection(LOD) and the limit of quantitation(LOQ) of undecylenic acid were 0.001 2% and 0.004%，respectively.The average recoveries of UA in cream，lotion，toner and sanitizer were between 93% and 104% with relative standard deviations(RSDs) of 0.39%-5.1%.