Introduction: In current practice the post-operative pain is managed in a routine manner without considering the individual patients need. Management of post-operative pain is one of the important concern of medical term in order to promote trust, comfort and reduce pain. Hence this study was undertaken to assess the effectiveness of Rhythmic Breathing Exercise on post-operative pain among patients undergone major abdominal surgery in MGMCRI at Puducherry. Objectives: To assess the level of post-operative pain among patients undergone major abdominal surgery. To assess the effectiveness of Rhythmic Breathing Exercise on post-operative pain among patients undergone major abdominal surgery. To associate the level of post-operative pain with their selected demographic variables. Methodology: The study was adopted with quantitative research approach and use pre-experimental (one group pre-test & post- test) design, 30 patients undergone major abdominal surgery was selected from MGMCRI at Puducherry by using purposive sampling technique and the data were collected by using demographic variables. The assessment of post-operative pain level was done by using numerical rating scale respectively as pre-test and post test with intervention of Rhythmic Breathing Exercise given to the patients undergone major abdominal surgery. Results: The level of post-operative pain among patients undergone major abdominal surgery during pre-test and post-test was assessed. In pre-test among 30 patients 28(93.3%) were having severe pain and 2(6.7%) were having moderate pain. In post-test 25(83.3%) were having moderate pain and 5(16.7%) were having severe pain. The effectiveness of Rhythmic Breathing Exercise on post-operative pain among patients undergone major abdominal surgery in the pre-test and post-test mean pain level was 7.83 and 5.70 the improvement was statistically tested by paired t- test which was found to be highly statistically significant at p <0.001 level it indicates that Rhythmic Breathing Exercise was highly effective in reduction of post-operative pain among patients undergone major abdominal surgery. Conclusion: Rhythmic Breathing Exercise is highly effective and easily can do and which is very beneficial for all patients undergone major abdominal surgery