The pyrolysis of geometrical isomers of 9-octadecenoic acid were conducted at $600^{circ}C$ under nitrogen, and its pyrolytic behaviors were investigated. The pyrolytic products of both fatty acids, cis-9-octadecenoic acid(oleic acid) and trans-9-octadecenoic acid(elaidic acid), were analyzed using a gas chromatograph and GC/MS spectrometer. Twenty-seven different compounds were identified from these pyrolyzates, and the chief products of them were ketones, alcohols and aromatic hydrocarbons. And also, the pyrolytic mechanisms of two geometrical isomers of this fatty acid were discussed by a comparison of experimental results.