Infrared absorption spectra of polyunsaturated fatty acid esters obtained from cuttle fish oils, showed a characteristic absorption at 1395cm-1, in addition to the typical absorption of normal fatty acid methylesters. This absorption at 1395cm-1 was related to the double bond, since by hydrogenation or autoxidation it was vanished away.Methyl linoleate, -linolenate and -arachidonate gave the suitable absorption at this wave number, while methyl oleate, -eleostearate and cyclododecatrien didn't.This characteristic absorption was assigned to the C-H bending vibration of the center methylene of divinylmethan radical (=CH·CH2·CH=).