Renal impairment and macrovascular endothelial dysfunction complicated with diabetes mellitus (DM) play a major role in the life expectancy of DM patients. Eucommia leaf extract (ELE) has anti-diabetic effects, but its effect on those complications with DM are unclear. Therefore, this study used the Zucker diabetic fatty (ZDF, type 2 DM) rat to examine the effects of ELE-containing diet (3% or 5%) ingestion before or after the onset of DM. Chronic ingestion of ELE prior to the onset of DM significantly suppressed the elevated blood glucose (BG) levels observed in ZDF rats and also remarkably improved glucose tolerance based on the results of the oral glucose tolerance test. Urinary protein excretion and albuminuria as important indicators of diabetic nephropathy (DN) were markedly suppressed by taking ELE 5% containing diet. A significant decrease in the acetylcholine-induced vasorelaxation response associated with DM was considerably improved by ELE ingestion. ELE ingestion after the onset of DM showed a reduction in BG levels, but no obvious effect on DN and endothelial dysfunction. These results indicate that chronic ingestion of ELE initiated before the onset of DM not only suppresses BG levels associated with DM progression but also has beneficial effects on DN and vascular endothelial dysfunction.