Background Berberine is one of the main constituents of Coptidis rhizoma (CR) and Cortex phellodendri, In this study, we investigated the beneficial effects of berberine on renal function and its possible mechanisms in rats with diabetic nephropathy(DN). Methods Male Wistar rats were divided into three groups: normal, diabetic model, and berberine treatment groups. Rats in the diabetic model and berberine treatment groups were induced to diabetes by intraperitonal injection with streptozotocin(STZ). Glomerular area, glomerular volume, fasting blood glucose(FBG), blood urea nitrogen(BUN), serum creatinine (Cr)and urine protein for 24 hours(UP24h) were measured using commercially available kits. Meanwhile, the activity of superoxide dismutase (SOD), content of malondialdehyde (MDA) in serum, activity of aldose reductase (AR)and the expression of AR mRNA and protein in kidney were detected by different methods. Results The result showed that oral administration of berberine (200mg·kg-1·d-1) significantly ameliorated the ratio of kidney weight to body weight. Glomerular area, glomerular volume, FBG, BUN, Cr and UP24h were significantly decreased in the berberine treatment group compared with the diabetic model group(P＜0.05). Berberine treatment significantly increased serum SOD activity and decreased the content of MDA compared with diabetic model group(P＜0.05). AR activity as well as the expression of AR mRNA and protein in the kidney was markedly decreased in the berberine treatment group compared with diabetic model group (P＜0.05). Conclusion These results suggested that berberine could ameliorate renal dysfunction in DN rats through controlling blood glucose, reduction of oxidative stress and inhibition of the activation of the polyol pathway.