Various kinds of free fatty acid were examined on their antitumor activity against ascites tumor cells, Survival days of mice after administrations of free fatty acid were observed. From these data I.L.S. (Increase of life-span) was culcurated and used as a parameter of Antitumor activity. Effective fatty acids against Sarcoma-180 were as follows : Pentadecanoic acid in saturated fatty acid series of carbon number 6 to 24, oleic acid in mono-unsaturated fatty acid series, and α-linolenic and docosahexaenoic acids in poly-unsaturated fatty acid series.<BR>The most effective fatty acid in our antitumor activity research is (hydroxy phenyl) stearic acid.<BR>The mechanism on antitumor activity of free fatty acid is not made clear sufficiently but it was suggested that disorder of membrane caused by fatty acids is responsible for their antitumor effect.