The right or whole thorax of Wistar rat was irradiated with 60Co ?-ray including single exposure (10,30,50,70Gy) and fractional exposures(30, 60Gy). Animals from each group were sacrificed at 0.5,1,2,3,6,9,12 months after irradiation. Sections of lung were examined with light microscopy (LM), electron microscopy (EM), histochemistry and morphometry. The pulmonary changes could be divided into four phases: exudation, granulation, fibroplasia and collagenation. The four phases can exactly show the development piocess of irradiation interstitial pneumonitis. Type Ⅱ pneumocyte and mast cell may play a role in pulmonary fibrosis formation.