Background: Atelectasis and hypoxemia are frequently reported after coronary artery bypass graft surgery (CABG). Some studies confirm the benefits of breathing exercises on pulmonary complications, but the efficacy of preoperative breathing exercises in patients undergoing CABG is controversial. In this study, the effect of preoperative breathing exercises on the incidence of atelectasis and hypoxemia in patients candidate for CABG was examined. Methods: In a single-blinded randomized clinical trial, 100 patients who were undergoing coronary artery bypass graft surgery were randomly allocated into two groups of experimental and control, each consisted of 50 patients. Before the operation, experimental group patients were enrolled in a protocol including deep breathing, cough and incentive spirometer. In the control group, hospital routine physiotherapy was implemented. All the patients received the hospital routine physiotherapy once a day for 2 to 3 minutes in the first four days postoperatively. Arterial blood gases and atelectasis were compared between groups. Results: There was no significant difference between groups in terms of atelectasis and hypoxemia (p Value>0.05). Conclusion: Preoperative breathing exercise does not reduce pulmonary complications in patients undergoing CABG.