Objective: To investigate the potential beneficial effect of bosentan in ameliorating fibrotic agents in diabetic mice. Materials and Methods: Male 6-week old C57BL/6 mice were divided into 3 groups (N=20): Control group, diabetes mellitus (DM) group and DM-B group (diabetes with bosentan group). Streptozotocin (STZ) was injected as 200 mg/Kg for single dose, i.p. (intraperitoneal injection). Fasting blood glucose (FBG) was measured at 0-, 1-, 2-week after STZ injection to confirm that diabetes was induced in the mice. Bosentan (100mg/Kg) and placebo was given i.g. (intragastric administration) once a day immediately after STZ injection for 18 weeks. The mRNA expression of tissue growth factor beta (TGF-b), connective tissue growth factor (CTGF), vascular endothelial growth factor (VEGF) and collagen-1 were evaluated by RT-PCR and real-time PCR. Differences in the data between the groups were compared by Student t-test for independent samples. Results: After 18 weeks of diabetic situation, FBG of DM-B mice was significantly higher than that of control mice and was similar with that of DM mice (DM mice vs. control mice, p<0.001; DM-B vs. control mice, p<0.001; DM mice vs. DM-B mice, p>0.05). The cardiac VEGF mRNA (a potent angiogenic factor) level in DM-B mice was significantly higher than DM mice (p<0.01). The heart of DM-B mice also showed lower expression of fibrotic genes (TGF-b, CTGF and collagen-1) than DM mice (p<0.01). Conclusion: These findings indicate the potential usefulness of an ET receptor antagonist bosentan in the amelioration of fibrotic agents, which may promote tissue fibrosis. This may provide a promising therapeutical strategy for diabetic cardiac fibrosis.