In this study diabetic patients who are candidated for vitrectomy with similar complexity scores will be randomized to preoperative injection or no injection of 2.5 mg Bevacizumab .In the injection group, 2.5 mg of bevacizumab (0.1 ml of commercially available Avastin vial, Genentech, inc. South San Francisco, CA) will be injected into the vitreous 3-5 days before operation.