Successful removal of cholesterol ester which is specifically accumulated in the arteriosclerotic region on the arterial walls was achieved by the exposure of 5.75 micrometers -infrared free-electron-lasers (FEL) which is absorbed by C equals O stretching vibrations of ester. Short pulse duration and high power density of FEL-micropulse and suitable wavelength absorbed by specific molecular vibrations may induced these non-thermal effects. However, details of interaction mechanism have not yet been clarified. In this paper, we discussed interaction mechanisms of biomolecules and IR lasers which excited molecular vibrations. Cholesteryl oleate were exposed to FELs and temperature increase was estimated for heating experiments which induce static thermal effects. Changes of the samples were examined with FT-IR. As results, it was found that FEL induces not only thermal effect by macropulse as observations of temperature increase, but also microscopic thermal effect by micropulse as observations of vaporization and IRMPD by wavelength of micropulse of 5.75 micrometers -FEL as observations of ester hydrolysis.