A new method of determination of fatty acid using gas chromatography-flame photometric detector (GC-FPD) is described. Fatty acid was methylthiomethyl-esterified with methylthiomethy chloride in 1,8-diazabicyclo [5,4,0]-undec-7-ene catalyst and its concentration measured by GC-FPD with 3%OV-1 or 3% FS-1 column. The FPD responses of thirteen fatty acid methylthiomethyl esters were examined and were proportional to the concentration of the esters without regard to their chemical structures. Consequently it was possible to determine various fatty acid using one standard calibration curve by GC-FPD. We could rapidly and conveniently determine varous fatty acids using one standard calibration curve by GC-FPD. We could rapidly and conveniently determine 13 fatty acids in Ginseng Radix alba by this method.