The refractive index of fluorinated polyimide is increased by electron beam irradiation. The degree of refractive index change can be controlled by adjusting the dose of the electron beam. The depth of the refractive index change depends on the energy of the beam. X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and surface profile measurement of electron beam irradiated fluorinated polyimide film were carried out to investigate the causes of the refractive index increase. The surface profile measurement indicated that the polyimide volume was decreased by an electron beam irradiation. The XPS and SIMS showed that electron beam irradiation decreases the fluorine content. The main cause of the refractive index increase is thought to be this decrease in fluorine content and volume.