Abstract Background: Post-mastectomy lymphedema is secondary lymphedema that impairs the lymphatic drainage system of the breast. Its signs and symptoms include increased weight and limb size. Since 60% of all lymph nodes are found right beneath the diaphragm, it serves as a lymphatic pump. By exerting a neg-ative pressure and drawing the lymph through the lymphatic system, diaphragmatic breathing (DB) promotes the cleaning of the lymph nodes. Aim of Study: The aim of the study is to evaluate deep resist-ed diaphragmatic breathing on post mastectomy lymphedema. Patients and Methods: Sixty eight patients with post mas-tectomy lymphedema were chosen at random and divided into two groups of equal size, (Group A) Resisted diaphragmatic breathing (DB) exercise and complete decongestive therapy (CDT) (pneumatic compression, compression bandage, man-ual lymphatic drainage, active range of motion AROM exer-cise and skincare), 3 session per week for 4 weeks, (Group B) received complete decongestive thereby (CDT) 3 session per week for 4 weeks. The participants were selected from Out Patient clinic. The study conducted six months from February 2022 to May 2023. Measurement: Patient's limb circumferential measurements were taken in five point (axillary, mid arm, elbow, mid for arm and wrist) for both group pretreatment and post treatment. Results: These findings reveal that there was a decrease in lymphedema measurement in both groups A and B when comparing post treatment and pre treatment measurement (p<0.001). However, Statistical analysis showed significant improvement in deep resisted breathing group in axillary, mid arm, elbow, mid forearm and wrist circumferential measure-ment of lymphedema in comparison to that of control group after treatment (p<0.001). Conclusion: Deep resisted diaphragmatic breathing exer-cise proved to be effective in decreasing lymphedema in post mastectomy patient.