Objective To study the effects ofautologous bone marrow mononuclear cells (BMMCs)transplantation during coronary artery bypass gafting (CABG) on myocardial infarct size. Methods Forty myocardial infarction patients diagnosed by coronary angiography (CAG) and SPECT and confirmed at surgery were enrolled and randomly assigned CABG alone (group Ⅰ) or CABG with intramyocardial or intracoronary injection of autologous BMMCs (group Ⅱ), 20 cases in each group. Baseline and followed up evaluations included SPECT and NYHA-FC before and after 6 months operation, recorded the major adverse cardiac events (MACE) at the same time. The number of autologous BMMCs injected was (6.84 ± 2.88) ×107 in group Ⅱ. Results There was no procedure-related complication during 6 months followed up in all patients. After 6 months operation,left ventricular ejection fraction in group Ⅱ was significantly higher than that in group Ⅰ [(57.40 ±5.21)% vs. (50.75 ±5.88)%,t =3.79,P＜0.05],NYHA-FC in group Ⅱ was significantly improved than that in group Ⅰ [(1.30 ± 0.47) grades vs. (1.85 ± 0.59) grades, t = 3.27, P ＜0.05],SPECT showed myocardial infarct size in group Ⅱ was significantly lower than that in group Ⅰ[(14.57 ±5.20)% vs. (20.45 ±5.18)% ,P ＜0.05]. Conclusion Autologous BMMCs transplantation during CABG is safe and feasible, which can reduce the myocardial infarct size in patients with myocardial infarction.