Aim The previous studies have demonstrated that the acetylcholine(Ach)-induced endothelium cells(EC)-dependent relaxation were injuried in diabetic models. The present study was to investigate the effect of Losartan, a specific inhibitor of angiotensin Ⅱ (Ang Ⅱ) receptor 1 (AT 1), on dysfunction of EC-dependent relaxation in diabetic rats induced by streptozotocin(STZ) at different time. Methods The Ach-induced EC-dependent relaxation of rat aortic rings were tested in tissue bath, the concentration of plasma glucose and Ang Ⅱ were measured. Results At 4 weeks, the function of EC-dependent relaxation was not injuried in diabetic group, but the concentration of plasma glucose and Ang Ⅱ were increased significantly compared with control group. The percent of Ach-induced relaxation was declined obviously compared with control group at 8, 16 weeks. Treatment with losartan significantly protected the function of EC-dependent relaxation, the percent of Ach-induced relaxation was obviously increased compared with diabetic group. The concentration of plasma glucose was not changed , but the plasma Ang Ⅱ was increased compared with diabetic group. Conclusion Losartan could protect the function of EC-dependent relaxation in diabetic rats.