To evaluate the expression of type I procollagen and type III procollagen mRNA by Qidan Granuless on Bleomycin induced pulmonary fibrosis in rats and to explore its pathogenesis, 160 SD rats in good conditions were randomly divided into six groups:normal control group, model group, Qidan Granules group(G1,G2) and hydrocortisonum group(H1,H2). Normal control group was treated with saline water. Pulmonary fibrosis was induced in rats by intratracheal instillation of Bleomycin (5 mg/kg body weight). 40 rats were chosen as model group and other rats were treated with Qidan Granules(3 125 mg/kg) or treated with Hydrocortisone(25 mg/kg)respectively. Rats of each group were sacrificed at 7, 14, 28 or 42 day post-Bleomycin and the lungs were obtained and fixed in paraformaldehyde and for in situ hybridization. All of the results were analysised by Leica Qwin V3 digital-image processing system. Qidan Granuless could obviously reduce alveolitis and fibrosis. III procollagen mRNA expression was increased in the acute alveolitis phrase of pulmonary fibrosis induced by Bleomycin, the expression of type I procollagen mRNA was mainly increased in the chronic stage of pulmonary fibrosis. The expression of III procollagen mRNA in Qidan Granules group was highest at 14 day and was higher than Hydrocortisone group(P<0.05). The expression of I procollagen mRNA was highest at 42 day, and there is a significant difference between Q1 and H1 group Q2 and H2 group at 28 day(P<0.05), but there is no significant difference between Q2 and H2 at 42 day. It is conclused Qidan Granules could obviously reduce alveolitis and fibrosis. Qidan Granules may alleviate the process of rat pulmonary fibrosis through influence the expression of I procollagen and III procollagen mRNA. The results strongly indicate the beneficial effects of Qidan Granules in protecting lung against Bleomycin-induced pulmonary fibrosis.