The present invention relates new use of rhein or the salt thereof, that is new use in preparing medicine for treatment of diabetic nephropathy. A medicine containing rhein or the salt thereof for treatment of diabetic nephropathy may be supplied in aqueous solution or capsula for oral, the dosage is not larger than 200 mg/day. Toxicity of rhein or the salt thereof is low, LD50 is 3.2 g/kg. The concentration of medicine in blood is still higher in 24 hours after administered rhein by oral. Rhein can effectively control or decrease the hyperglycemia, suppress renal hypertrophy and kidney index in experimental diabetic rats. The urinary protein execretion was significantly decreased in the diabetic patients treated by rhein.