Abstract: A method of GC-MS for determination of 4 fatty acids, i.e., palmitic acid, stearic acid, oleic acid and linoleic acid was proposed. The sample was derivatized with methanol in H2SO4 medium, and the derivate was dissolved with n-hexane. J&W DB-WAX capillary column was used for GC separation and the full-scanning mode and selected ion monitoring mode were adopted in MS determination. Linear relationships between values of peak area and mass concentration of 4 fatty acids were obtained in definite ranges, with limits of detection (3S/N) and lower limits of determination in ranges of 4.42-22.5 mg·L-1 and 14.7-75.0 mg·L-1 respectively. The proposed method was used in determination of fatty acids in intermediates and leftover in the process of preparation of mixed fatty acids.