This paper is an attempt to describe the role of the radiation of the far．infrared ray in oxidation of the PAN fiber by heating in air．Oxidation．effect of the PAN fiber by heating in air under radiation of the far-infrared ray has been investigated by infrared spectroscopy，DSC，DTA，elemental analysis，X-ray and electron microscopy techniques．The analytical results indicate that radiation of the Farinfrared ray ef-fectively promotes oxidizing·cyelizing reaetions of the PAN fiber by heating in air．The most important characterisitics of the oxidized fiber (by heating in air with radiation of the far-infrared ray) appear to be the presence of more oxygen and the formation of oxidized-cyelized more thick sheath．During oxidation，unsaturation，hydroxyl and carbonyl groups are introduced，and thermally stable ladder polymer is formed．