Wistar rats (4-week-old) were administered with streptozotocin (45 mg/kg) through tail veins. After 3 months, diabetic rats were divided into 2 groups. One group (EPA group, n = 16) was fed a lipid-free diet (90%, w/w) plus lard (8%) and 90% pure eicosapentaenoic acid ethyl ester (2%) for 6 months. The other group (control group, n = 16) was fed in the same way except that eicosapentaenoic acid ethyl ester was replaced by safflower oil. Twenty-four-hour urine was collected just before starting the experimental diets and during the 6-month experimental period at monthly intervals. There were no differences in food intake and body weight between the two groups throughout the experiment. The mean microalbuminuria of the EPA group became significantly lower than that of the control group after 4 months on the diets through the end of the study (6 months). The mean microalbuminuria levels at the end of the study were 1.38 mg/day in the EPA group (n = 9) and 5.19 mg/day in the control group (n = 6) (p < 0.01). Eicosapentaenoic acid administration might retard the progression of diabetic nephropathy by reducing microalbuminuria.