Abstract: Objective:To study the pathological and ultramicrostructural characteristics and etiology in relation to the clinical course of Severe Acute Respiratory Syndrome（SARS）.Methods:Post-mortem tissue samples of mult-i organs （lungs，heart，liver ，spleen，kidneys，pancreas，stomach ） were taken by needle biopsy from four SARS patients who died at 3-5weeks of middle and late stages .Systematic autopsy were carried on two SARS cases who died at 1-2weeks of early stages.The pathological samples were studied by light and electron microscope，immuno-histochemistry，histochemistry and indirect immunofluorescent antibody test.Results:Electron microscopy showed the coronavirus-like particles with the diameter of 60-220nm and polymorphism. The chlamydia like particles and suspicious mycoplasma-like particles were observed in the lung and other organs.The main pathological features of 1-2weeks of early stages of SARS were diffuse alveolar damage （DAD） with pulmonary edema and desquamative alveolitis .Hyaloid Membranes were formed extensively.The main pathological features of 3-5weeks of middle and late stages of SARS were the early interstitial pulmonary fibrosis or organizing pneumonia.Fibroblasts increased in the interalveoli septa and young connective tissue filled alveoli.Diffuse alveolar damage and desquamative alveolitis also existed. Conclusions:There were the different pathological features in the different stages of SARS.