Background: Postoperative pulmonary complications (PPCs) are common after cardiac Surgery. They are a major contributor to morbidity, compromise oxygenation, extend hospitalization, and raise death. Aim: The present study aimed to evaluate the effect of deep breathing exercises on pulmonary functions after cardiac surgery. Subjects and Method: Design: A Quasi experimental design was used. Setting: The study conducted in intensive care units of cardiac surgery and surgical wards settings at Al-Nasr specialized hospital in Port Said Governorate and Damietta cardiac and gastrointestinal center in Damietta Governorate. Subjects: It included a purposive sample of 74 patients post-operative cardiac surgery. Tools: Four tools were used for data collection consisted of; medical and surgical history assessment in addition to patient sociodemographic characteristics, Pulmonary functions assessment, Melbourne Group Scale Version II, and Observational checklist for deep breathing exercises. The Results: There were statistically significant differences between pre and post applying deep breathing exercises regarding all pulmonary functions after cardiac surgery with P-Value of (<0.001). Conclusion: There was statistically significant relation between the studied patients' BMI, education, and health habits and post patients` education regarding applying deep breathing exercises. Furthermore, there was statistically significant relation between deep breathing exercises and pulmonary functions after cardiac surgery. Recommendations: Further research is necessary to design and implement educational programs for critical units' nurses regarding importance of applying deep breathing exercises for patients after cardiac surgery to enhance pulmonary functions and prevent pulmonary complications.