Lactuca sativa L. (Garden Lettuce), is an edible herb cultivated in Iran and other parts of the world. In Traditional Iranian Pharmacy books, garden lettuce is named "Kass Bostani" and Hakim Aqili classified it as a "Ghazā'ye Dawā'ee" (Ghazā means Food; Dawā means Drug). It is said to be soporific, prescribed to cure insomnia and to be useful in thirst and feeling of hotness and burning in the stomach. Seeds of this herb reduce semen, suppress libido and are useful in cases of frequent nocturnal emissions. Fixed oil obtained from seeds of this plant is reputed to have hypnotic and brain moistening properties. In this study we aimed to analyze the fatty acid composition of the crude seed oil of Lactuca sativa L. Methyl esterification of the fatty acids was performed by the method of Ken'ichi Ichihara et al but with a slight modification. Components of the oil were then extracted by n-hexane and analyzed by Gas chromatography-Mass spectroscopy and Gas Chromatography methods. Identified constituents which represented 98.20% of the total elutes were the methyl esters of linoleic (52.38%), oleic (34.42%), palmitic (7.25%), stearic (2.66%), arachidic (1.32%) and myristic (0.17%) acids. Total percentages of methyl esters of the saturated and unsaturated fatty acids identified in our examined oil are 11.4 and 86.80% respectively. In conclusion, seed fat of Lactuca sativa L. like many other plant fats, is rich in unsaturated fatty acids.