This study was designed to investigate a natural medicinal multi-plants extract (BG515), which consisted of multi extracts of Mori folium, Rehmannia glutinosa Liboschitz, Dioscorea japonica, Lycii fructus, and Astragalus radix, on blood glucose, insulin levels, and serum malondialdehyde (MDA) concentrations in streptozotocin-induced diabetic rats. Streptozotocin (STZ) induces a type 1 diabetes mellitus in rats. Type 1 is usually characterized by the presence of islet cell autoantibodies (ICA), autoantibodies to insulin (IAA), and autoantiboides to glutamic acid decarboxylase (GAD), which identify the autoimmune process that leads to $eta-cell$ destruction. Thirty-five male Sprague-Dawley (SD) rats weighing $150sim170g$ each (6 weeks old) were randomly divided into one control (Group A) and 4 STZ-induced diabetic groups, and were subjected to one of the following treatment for 12 weeks. Groups A and B were fed basal diets and Group C, D, and E received the same diets as groups A and B, but with supplements of 150 mg/kg, 300 mg/kg, and 600 mg/kg of BG515 orally for 12 weeks, respectively. Diabetes was induced in Groups B, C, D, and E by intravenous injection of 45 mg/kg of STZ per body weight in sodium citrate buffer (pH 4.5) via the tail vein. In the BG515 groups, we found increases in serum insulin levels, compared to the STZ-control group, but these data were not significant. In contrast, blood glucose and serum MDA concentrations decreased in the BG515 groups compare to the STZ-control group. At the 5th week, in all the BG515 administered groups, there were decreases in serum blood glucose levels compared to the STZ- control group, and this activity was very strong in the BG515-1 group at the 12th week. These results suggest that natural bio-complex compounds (BG515) may slightly suppress STZ-induced changes in serum MDA concentration via the maintenance of serum insulin levels, due to the prevention of $eta-cell$ and glucagon destruction by STZ.