Purpose: The aim of this study was to evaluate the effect of recruitment maneuvers (RMs) and positive end expiratory pressure (PEEP) on diaphragmatic function and atelectasis areas during pneumoperitoneum and the trendelenburg position in laparoscopic sleeve gastrectomy (LSG) patients. Methods: This prospective double-blinded randomized clinical study was conducted on 69 obese patients undergoing LSG. Patients were randomly allocated to one of the three groups, Group I (control group) patients were mechanically ventilated without PEEP or RM, Group II received PEEP of 5 cmH2O, and Group III received 5 cmH2O PEEP, and intermittent four times RM consisting of maintaining airway pressure 40 cmH2O for 40 s. Primary outcome was diaphragmatic excursion (DE). Secondary outcomes were atelectasis area, Forced Vital Capacity (FVC), Forced Expiratory Volume in 1 s (FEV1), Peak Inspiratory Pressure (PIP), and any complications. Results: DE decreased in all investigated groups after anesthesia induction till the end of procedure compared to pre-induction values, and it was statistically significant lower in control group compared to the two interventional groups with no significant difference between PEEP and PEEP+RM groups. Atelectasis volume according to lung aeration score was significantly increased in control group compared to the other two groups, while there was no statistical significant difference in PEEP+RM compared to PEEP in all the times except before induction of anesthesia there was no significant difference among the three groups. Conclusion: The application of RM and PEEP are helpful for preserving DE and improving lung aeration during laparoscopic sleeve gastrectomy.