Background: the diabetic type 2 population development in the world wide. Its risk describe through the long term complication that provide in destroyed the variety tissues in organs as liver and blood vessels. Cinnamon zeylanicum classes as famous medicinal herb , contain large numbers of antioxidant phytochemical compounds: (cinnmaldehyde ,eugenol ,gallic acid, rutin) and others phenolic compounds and vitamins as(A,K,E,D3,K,C,B1,B2) ,also researches indicated that cinnamon contain interesting minerals like ( zinc, copper, Iron, cobalt).therefor cinnamon have capacity to treated T2DM . Aim: present study aimed to investigation in vitro and in vivo antioxidant effect of aqueous bark extract of C. zeylanicum on oxidative stress in T2DM induce by Streptozotocin (STZ) in mice .Methodology: DPPH free radical scavenging use to evaluated antioxidant activity in vitro. In vivo: Eighty adult male, albino mice were be used and divided into four (n=20).group (A) normal healthy control mice. group (B) received (200mg/kg)cinnamon aqueous bark extract only. Group (C)mice fed with high fat diet for three weeks and received with (40 mg /kg) STZ injection (diabetic mice group).group (D) firstly treated by the same condition of group (C) and after (10 days) of STZ injunction the diabetic mice treated with (200 mg /kg) of cinnamon extract . Hyperglycemia was investigation by FBS and oxidative stress was investigation by permeant ROS, MDA, TOS and the total antioxidant capacity TAC in serum . Rustle: cinnamon zeylanicum extract showed effective DPPH free radical. Control group(A) compared T2DM group (C) and treatment group (D), herb caused a significant (p≤0.001) reduction in ROS,MDA,TOS, and a significant increase in total antioxidant capacity TAC also showed significant improvement in FBS and serum insulin. Conclusion: cinnamon zeylanicum a protective effect against oxidative stress in diabetic mice due to its antioxidant characters.