Abstract Background: Autonomic complications are among the most frequently reported complications after Coronary Artery Bypass Graft (CABG) surgery. However, the risk of autonomic complications and it's consequences is not equal for all patient, pre-operative, intraoperative and post-operative management modify the autonomic nervous system. HRV might assume an important role as a potential detector of autonomic system instability in the early post-operative phase in Intensive Care Unit and during hospital stay. Aim of Study: The aim of this study was to investigate the effect of using inspiratory muscle trainer as a method of deep breathing exercises rehabilitation, on autonomic nervous system modulation by analyzing heart rate variability and on functional capacity through 6-minute walk test in phase 1 cardiac rehabilitation post coronary artery bypass graft (CA-BG). Material and Methods: Forty male patients who underwent CABG aged from 40-60 years old and recruited from the Cardiothoracic Surgery, Kasr El-Ainy Hospital, Cairo Univer-sity enrolled in that study. They were assigned into two groups: Group (A) studying group: Consisted of 30 patients, which received inspiratory muscle training by inspiratory trainer in addition to routine physiotherapy programme according to department protocol in for of, percussion, vibration, and breathing exercises). Group (B): Controlled Group: The control group consisted of 20 patients, which received routine chest physiotherapy program, percussion, vibration, and breathing exercises). Programme started at first day after weaning from mechanical ventilation and end at the day of discharge. Results: Data obtained from two groups at the beginning of the study and after the end of the study before discharge from hospital regarding root square of successive differences between RR intervals (RMSSD) to calculate heart rate varia-bility and 6 minute walking distance were statistically analyzed and compared within the same group and between the two groups. Group A showed a significant increase in the RMSSD, and walking distance through 6 minute compared with that of group C (p<0.05) within the same group, and between the two groups. Using inspiratory muscle trainer in phase 1 cardiac rehabilitation post Coronary Artery Bypass Graft (CABG) is effective to enhance vagal tone through increasing heart rate variability and functional capacity. Conclusion: Within the limitation i of the current study, it is concluded that, inspiratory muscle training as a method of deep breathing technique ay improve heartrate variability as an indicator for vagal activity, that in turn improve cardiac sympathovagal balance, it was also included that the deep breathing by inspiratory trainer will also improve functional capacity which measured by distance travelled in 6 minute walk test.