Successful removal of cholesterol ester which is specifically accumulated in the arteriosclerotic region on the arterial walls was achieved by the exposure of5.75tm-infraredfree-electron-1asers (FEL) which is absorbed by C=O stretching vibrations of ester. In this study, chemical bond-changes and decrease in amount of cholesteryl oleate in IRFEL photodynamic processes were examined by Fourier transform infrared microspectroscopy (FT-IR). Wefound that the exposure of 5.75tm-FELs induces not only dissociation of ester (RCOOR') bonds but also the oxidation process of the ester (RCOOR') bonds to aldehyde(RCHO), carboxylic acid (RCOOH), carboxylate (RCOO) or ketone (R2C=O) bonds. Further, we exposed cholesteryl oleate to FELs having wavelength of5.75tm, 9.4ORm and8.44tm. From changes in absorbance andthickness of the samples, we calculated quantum efficiency 1 for decrease of ester bonds and C-C chains for each wavelength-EEL exposure. 1 was obtained to be 2.1x104 forester bondand 1.2x1Ofor C-Cchainby 5.75.tm-FELs,O.9x1O and2.0x104 by 9.4Otm-FELs, and 1.2x104 and 1 .0x104 by 8.44m-FELs. Thus, we canconcludethat removal ofcholesteryl oleate by IR-FELs has the wavelength sensitivity coming from correspondence between the bonds which absorbs FEL photons and which aimed to remove.