We investigated the long term effects of Sopungsungi-won (SP), a Korean traditional formula used for senile constipation and diabetes mellitus, on the development of diabetic nephropathy (DN) in Zucker diabetic fatty (ZDF) rats. ZDF rats were fed regular laboratory chow mixed with SP or rosiglitazone (RSG) for an 8-week period. Kidney hypertrophy was developed with increasing plasma glucose level, and glomerular hypertrophy was improved by 22% and 45% in SP- and RSG-treated rats, respectively. Urinary glucose and albumin excretions were also significantly lower in SP-treated rats than in ZDF control rats. Activation of the mitogen-activated protein kinase (MAPK)-transforming growth factor ${eta}1$ (TGF ${eta}1$)-fibronectin pathway in kidney, responsible for glomerular dysfunction, was markedly blunted by SP treatment in a dose dependent manner. Our findings, for the first time, provide strong evidence that long-term administration of SP formula prevents the development and progression of DN in ZDF rats. Human trials are needed to confirm these experimental results.