In order to examine the process of the lipid oxidation, the regioisomers of hydroperoxides produced by the photosensitized peroxidation of cholesteryl oleate, cholesteryl linoleate and cholesteryl Linolenate by photosensitized peroxidation and the separation of each isomer by HPLC were investigated. The olefinic parts in fatty acid chains of the above cholesteryl esters received hydroperoxidation predominantly by photosensitized oxidation. Cholesteryl oleate was subjected to hydroperoxidation at the 9 and 10 position of the oleic acid chain. Cholesteryl linoleate and cholesteryl linolenate were also subjected to hydroperoxidation at the 9, 10, 12 and 13 positions of the linoleic acid chain and at the 9, 10, 12, 13, 15 and 16 positions of the linolenic acid chain, respectively. Hydroperoxidation on the cholesteryl ring was also observed, however, their formation was negligibly small compared with hydroperoxidation of the fatty acid chain. The elution order of isomeric peroxides in HPLC was determined using a normal-phase column (Mightysil 60) eluted with hexane/2-propanol by the detected wavelength at 210 and 233 nm.