Objectives: Diabetes mellitus is one of the major risk factors for cardiovascular disease, and increases the risk of postoperative mortality. HbA1c is a measure of glycemic control. This study aimed to determine the effect of glycemic status on the complications of coronary artery bypass graf (CABG) surgery in diabetic patients. Methods: This cross-sectional study was performed on 300 diabetic patients including 186 (62%) men and 114 (38%) women who underwent CABG from March 2010 to August 2011 and selected via census method. Preoperative blood glucose control status was determined using HbA1c levels, and the patients were evaluated for postoperative in-hospital complications. Results: The mean age of the patients was 62.3±  6.9 years. In-hospital mortality was 1.0% (3/300). New-onset atrial fibrillation was the most common complication (35%). HbA1c level above 7.5% was a predictor of in-hospital mortality after CABG (unadjusted odds ratio 1.9, 95٪ CI: 1.1-3.2, p = 0.006). After stratifying analysis with Chi-sqaure test we found that gender history of myocardial infarction had no significant relationship with HbA1c level in two groups with or without complication. Conclusions: Long-term uncontrolled blood glucose before CABG is associated with increased postoperative complications. Preoperative HbA1c measurement can be a predictor of high-risk patients.