Background: Pulmonary complications following valvular heart surgery are common and contribute to increased duration of hospital stay, rate of morbidity, and mortality. The purpose of the present study was to investigate the response of maximum inspiratory pressure and functional capacity to Positive End-Expiratory Pressure device in patients who underwent valvular heart surgery. Materials and Methods: Thirty males and females who underwent valvular heart surgery aged from 12-18 years old and recruited from the National Heart Institute enrolled in this study. They were assigned into two matched groups: the intervention group consisted of 15 patients, received Positive End-Expiratory Pressure (PEEP) with the mouthpiece in addition to routine chest physiotherapy program; the control group consisted of 15 patients, received routine chest physiotherapy program only. The program continued for four weeks, then the results compared in two groups. Results: According to the results of data analysis, there was a statistically significant difference between pretreatment and post-treatment data in both groups regarding inspiratory muscle strength and functional capacity (p< 0.001). There was no significant difference between groups regarding post-treatment data of Maximal Inspiratory Pressure (MIP), and VO2 max (p=0.084, p=0.325), respectively. Conclusion: According to the results, expiratory training using a PEEP device with mouthpiece improved inspiratory muscle strength and functional capacity after valvular heart surgery.