OBJECTIVE To study the renal protective effect of sodium ferulate (SF) and its mechanism in rats with diabetic mellitus (DM). METHODS DM rats induced by streptozotocin were treated with SF 110 mg/kg per day for 8 weeks. The ratio of kidney weight/body weight (KW/BW), serum triglyceride (TG) and total cholesterol (TC), creatinine clearance rate (Ccr), urinary protein/24 hrs, levels of endothelin-1 (ET-1) and nitric oxide (NO) in renal cortex in rats were measured, the pathological change of kidney were observed and the expression of transforming growth factor-beta 1 (TGF-beta 1) and collagen IV (C-IV) in kidney were examined using immunohistochemical assay. The data obtained were compared with those obtained from untreated DM rats and normal rats respectively. RESULTS Compared with the normal rats, in DM rats, Ccr, urinary protein/24 hrs, ET-1, expressions of TGF-beta 1 and C-IV were significantly increased in DM model rats (all P < 0.01), and significantly abnormal pathological change in kidney was found. While in the SF treated DM rats, the above-mentioned abnormal changes were all significantly improved. CONCLUSIONS SF has effect in protecting kidney of DM rats, the mechanism might be related with its actions of reducing ET-1 production in kidney and inhibiting the expressions of TGF-beta 1 and C-IV.