The photooxidation of linoleic acid (LA) in the presence of methylene blue as a photosesitizer and inhibitory behavior of various antioxidants in vitro were investigated. Under air supply, LA that irradiated with visible light or red light was photooxidized only by addition of methylene blue. In the presence of methylene blue, the degree of photooxidation of LA with visible light was about twice as much as that with red leght. The photooxidation of LA in ethanol solution to which BHA (butylated hydroxyanisole), BHT (butylated hydroxyto-luene) and dl-a-Toc (dl-α-tocopherol) was added to be 0.1% in final concentration, BHA was inhibited proportionally to the concentration of BHA, but in addition to dl-α-Toc (dl-α-tocopherol) was inhibited when it`s concentration was higher than 0.1%. When methylene blue was added to ethanol solution, BHA itself was little oxidized, but dl-α-Toc was at the concentration between 10^(-3)%∼10^(-1)% and BHT was highly oxidized according to the increase of concentration. In the case of antioxidants was added to chloroform-ethanol solution containing LA to be 10^(-4)% in final concentration, the photooxidation of LA was significantly inhibited by both β-carotene and dl-α-Toc but β-carotene was effective more than dl-α-Toc. Also BHA and BHT were not effective.