Objective:To investigate ultrastructural changes in pulmonary tissue of a rat model of pulmonary fibrosis following treatment with compoundCarapax trionycis(C. trionycis;Biejia inChinese) formula.Methods:Sixty maleSprague-Dawley rats were randomly divided into four groups(n=15): compoundC. trionycis formula high-, middle-, and low-dose groups as well as model group. Pulmonary fibrosis was induced by bleomycin.Five rats from each group weresacrificed on day 7,14 and28 of the drug treatment, respectively.The pulmonary tissue was harvested followed by hematoxylin-eosin staining and subsequent transmission electron microscopy.TheSzapiel’s method was used to assess the degree of alveolitis and pulmonary fibrosis.Results:Compared with the model group, the compoundC. trionycis formula groups had slighter pulmonary alveolitis after the7-day treatment and also had alleviated alveolar inflammation and pulmonary fibrosis after the14-day treatment.After the28-day treatment, the compoundC. trionycis formula groups showed deposition of a small amount of fibrous tissue and lesions occupying less than21% of the whole lung area, while the model group showed focal or diffuse fibrous deposition, narrow alveolar cavity, disordered lung structure, and lesions in larger than51% of the whole lung area. Conclusions:The compoundC. trionycis formula can inhibit the proliferation of collagen fibers and resist pulmonary fibrosis.