Objective: To explore the role of 5-hydroxytryptamine (5-HT) in type 2 diabetes mellitus (T2DM)-related hepatic inflammation and fibrosis. Methods: Male C57BL/6J mice were used to establish T2DM model by high-fat diet feeding combined with intraperitoneal injection of streptozotocin. Then, the mice with hyperglycemia were still fed with high-fat diet for nine weeks, and treated with or without 5-HT_2A receptor (5-HT_2AR) antagonist sarpogrelate hydrochloride (SH) and 5-HT synthesis inhibitor carbidopa (CDP) (alone or in combination). To observe the role of 5-HT in the myofibroblastization of hepa-tic stellate cells (HSCs), human HSCs LX-2 were exposed to high glucose, and were treated with or without SH, CDP or monoamine oxidase A (MAO-A) inhibitor clorgiline (CGL). Hematoxylin & eosin and Masson staining were used to detect the pathological lesions of liver tissue section, immunohistochemistry and Western blot were used to analyze protein expression, biochemical indicators were measured by ELISA or enzyme kits, and levels of intracellular reactive oxygen species (ROS) were detected by fluorescent probe. Results: There were up-regulated expressions of 5-HT_2AR, 5-HT synthases and MAO-A, and elevated levels of 5-HT in the liver of the T2DM mice. In addition to reduction of the hepatic 5-HT levels and MAO-A expression, treatment with SH and CDP could effectively ameliorate liver lesions in the T2DM mice, both of which could ameliorate hepatic injury and steatosis, significantly inhibit the increase of hepatic ROS (H₂O₂) levels to alleviate oxidative stress, and markedly suppress the production of transforming growth factor β1 (TGF-β1) and the development of inflammation and fibrosis in liver. More importantly, there was a synergistic effect between SH and CDP. Studies on LX-2 cells showed that high glucose could induce up-regulation of 5-HT_2AR, 5-HT synthases and MAO-A expression, increase intracellular 5-HT level, increase the production of ROS, and lead to myofibroblastization of LX-2, resulting in the increase of TGF-β1 synthesis and production of inflammatory and fibrosis factors. The effects of high glucose could be significantly inhibited by 5-HT_2AR antagonist SH or be markedly abolished by mitochondrial 5-HT degradation inhibitor CGL. In addition, SH significantly suppressed the up-regulation of 5-HT synthases and MAO-A induced by high glucose in LX-2. Conclusion: Hyperglycemia-induced myofibroblastization and TGF-β1 production of HSCs, which leads to hepatic inflammation and fibrosis in T2DM mice, is probably due to the up-regulation of 5-HT_2AR expression and increase of 5-HT synthesis and degradation, resulting in the increase of ROS production in mitochondria. Among them, 5-HT_2AR is involved in the regulation of 5-HT synthases and MAO-A expression.