Objective:To explore the effects of melatonin on streptozotocin(STZ)-induced diabetic pulmonary fibrosis and regulatory mechanisms.Methods:C57BL/6 mice were randomly divided into the control group, STZ group, STZ+ low-dose melatonin(5 mg/kg) group, STZ+ high-dose melatonin(30 mg/kg) group using random number table, and a single intraperitoneal injection of STZ(150 mg/kg) was administered to establish a diabetic pulmonary fibrosis mouse model. Two weeks later, blood glucose levels ≥16.7 mmol/L confirmed successful modeling. Subsequently, melatonin was administered orally for 4 weeks, and the mice were sacrificed at 16 weeks for tissue sampling. Human umbilical vein endothelial cells were divided into the control group(glucose concentration is 5.5 mmol/L), high glucose group(glucose concentration is 33.3 mmol/L), high glucose+ low-dose melatonin(5 μmol/L) group, high glucose+ high-dose melatonin(20 μmol/L) group, and cells in each group were collected for subsequent detection after drug stimulation. Masson staining and immunofluorescence staining were used to observe fibrotic lesions, Western blotting was used to detect the expression related proteins, and sirtuin 3(Sirt3) siRNA was transfected to knock down Sirt3.Results:Significant fibrotic lesions were observed in the lung tissue of the STZ group compared to the control group, however, the STZ+ low-dose melatonin group and STZ+ high-dose melatonin group showed reduced fibrosis compared to the STZ group. In addition, compared to the control group, the endothelial cell marker platelet endothelial cell adhesion molecule-1(CD31) was significantly decreased in the STZ/high glucose group( P<0.001; P<0.001), and the interstitial fibrosis markers collagen 3, Vimentin, and α-smooth muscle actin(α-SMA) were significantly increased( P<0.001, P=0.035, P<0.001; P<0.001, P<0.001, P<0.001), but these trends were partially reversed after melatonin treatment in the STZ/high glucose+ low-dose melatonin group and the STZ/high glucose+ high-dose melatonin group. Moreover, the protein expression of Sirt3 was significantly reduced in the STZ/high glucose group compared to the control group( P<0.001; P<0.001), while it was increased in the STZ/high glucose+ low-dose melatonin and STZ/high glucose+ high-dose melatonin groups compared to the STZ/high glucose group( P=0.047, P<0.001; P=0.048, P<0.001). After transfecting Sirt3 siRNA to knock down the expression of Sirt3, the endothelial cell marker CD31 was significantly reduced( P=0.026), and interstitial fibrosis markers collagen 3, Vimentin, and α-SMA were significantly increased in the high glucose+ high-dose melatonin+ Sirt3 siRNA group compared to the high glucose+ high-dose melatonin group( P<0.001, P<0.001, P<0.001). Conclusion:Melatonin inhibits endothelial-mesenchymal transition by activating Sirt3 expression, thereby alleviating pulmonary fibrosis in STZ-induced diabetic mice.