Background: Coronary artery bypass graft (CABG) surgery with cardiopulmonary bypass (CPB) triggers an inflammatory reaction, leading to the development of myocardial damage and dysfunction. Selenium is the main cofactor for many antioxidant enzymes. Selenium level is decreased during heart surgeries affecting the cardiopulmonary pump, which in turn can aggravate the organ and heart dysfunction and mortality. Objectives: The aim of this study was to evaluate the cardiac protective effects of adding selenium to cardioplegia solution in these surgeries. Methods: In this randomized, double-blind, clinical trial study that was conducted in the department of cardiac surgery of Shiraz University of Medical Sciences (SUMS) in Shiraz, Iran, 67 elected CABG patients were allocated to the two control or selenium groups.In the intervention group, 1000 µg sodium selenite was added to cardioplegia solution. The same amount of normal saline was added to the cardioplegia solution in the control group. Arterial blood samples were withdrawn before anesthesia induction (T1), immediately after the surgery (T2), as well as, 6 and 24 hours after the surgery (T3 and T4 respectively), to determine the CK-MB and Troponin I levels.Results: According to our findings, the CK-MB and Troponin I cardiac enzyme levels were significantly different, considering different time points (P < 0.05). Despite lower enzyme levels in the selenium group, the differences were not statistically significant between the two groups (P > 0.05). There were also no significant differences between the two groups regarding systolic and diastolic blood pressures. Conclusions:The administration of 1000 µg sodium selenite via cardioplegia solution had no significant cardioprotective effect during coronary bypass surgery in CABG patients.