Introduction: Left ventricular ejection fraction (LVEF) is considered to be the single most important prognostic factor in patients with previous myocardial infarction. LVEF is not improved in all patients after coronary artery bypass grafting (CABG). This study aimed to assess the possibility of prediction of LVEF changes after CABG using myocardial perfusion gated signle photon emission computed tomography (GSPECT). Materials and Methods: Overall, 48 patients with mean LVEF of 30.2% (±4.7) underwent Echocardiography and GSPECT after injection of Tc-99m-MIBI at rest. Myocardial uptake was evaluated in 17 myocardial segments and was compared with age and gender matched normal data pool. The risks and benefits of CABG were explained to the patients and 16 cases (15 male and 1 female) with the mean age of 61.1 years (±10.8) accepted to undergo off-pump CABG. All the patients were followed-up for at least six months and echocardiography and GSPECT were repeated at the end of follow up. Results: The mean LVEF was increased from of 31.1% (±3.5) to 34.5% (±3.6) after surgery (P<0.001). Delta LVEF was defined as ΔLVEF=LVEF (before CABG) - LVEF (after CABG). ΔLVEF was within the range of 0-8% with the mean of 3.4% (±2.5). The number of non-viable myocardial segments was not significantly different between patients with ΔLVEF ≥ 5% and those with smaller changes. Myocardial perfusion was estimated for all segments, and the mean global uptake was defined by adding the mean uptake in all segments, divided by 17. The mean global uptake was 53.1% in our patients. Regression analysis revealed that ΔLVEF after CABG can be predicted reliably using the following formula: ΔLVEF= -33.8 + (0.77 × mean global uptake) (P<0.01). Conclusion: Our study showed that change of LVEF after CABG can be predicted reliably using mean global uptake in preoperative myocardial perfusion SPECT at rest.