Objective: We compared electron microscopic histologic changes of the radial artery grafts in non-diabetic and diabetic patients. Methods: Thirty-six patients were divided into three groups according to their diabetic status (Group I had no diabetes mellitus [DM], Group II had type two DM and HbA1c levels were <7.5%, and Group III had type 2 DM but HbA1c levels were >7.5%). Distal parts of radial artery grafts were evaluated with scanning electron microscopy in a blind fashion by two histologists. Electron microscopic scores were compared among the groups. Results: Radial artery electron microscopic scores were significantly different between group 1, 2 and 1, 3 and 2, 3 (p = 0.028, p<0.001, and p<0.001). In linear regression analysis, duration of DM (p = 0.027) and fasting plasma glucose (p = 0.001) were found as independent risk factors for histologic changes of radial artery grafts. Conclusion: Duration of DM and poor glycemic control were found to be associated with radial artery electron microscopic changes.