Abstract: It aimed to investigate the negative effect of cardiopulmonary bypass on pulmonary function tests in early postoperative term by comparing pre and postoperative pulmonary function tests and level of blood gases. About 134 consecutive patients underwent CABG surgery between January 2008 and November 2009 were enrolled to prospective study. Patients were divided into two groups: group 1 (n = 67) included patients who urderwent on-pump cardiac surgery, group 2 (n = 67) included patients who underwent off-pump cardiac surgery. Pulmonary function tests were recorded preoperatively and at postoperative 6th day. The follow-up parameters including Vital Capacity (VC), Forced Expiratory Volume at 1 sec (FEV₁) and FEV₁/VC% of patients in both groups were compared. In present study, 78 males and 56 females overall 134 patients (mean age 62.2±6.4) were evaluated. Overall mortality was 2.98%. Although, there was no difference in FEV₁ level in preoperative period, it was higher with statistical significance in off-pump group compared on-pump group (77.1±4.2 vs. 63.2±4.2, p<0.005, respectively) at postoperative 6th day. FEV₁/VC rate was also higher with statistical significance in off-pump group than on-pump group (88.1±12.1 vs. 66.4±14.2, p<0.001, respectively) at postoperative 6th day while there was no difference in preoperative period. It think that the off-pump technique is more usufel not only in high risk patients but also in all patients will undergone CABG surgery when the negative effect of CPB just on pulmonary function tests is considered.