Background: Breast Cancer is the most commonly diagnosed cancer among women worldwide. Simple mastectomy and modified radical mastectomy are the preferred surgery to treat breast cancer. Patients undergone mastectomy experience arm lymphedema symptoms which affects their physiological as well as psychological well-being. The main objective of this study was to assess the effectiveness of discharge teaching regarding post mastectomy care on self-care practice and arm lymphedema among patients with breast cancer undergone mastectomy. Methods: Quantitative research approach with quasi-experimental time series design was used in this study. Fifty-two patients with breast cancer were selected by consecutive sampling technique. Baseline data on selfcare practice and arm lymphedema symptoms were collected on 1st postoperative day. Day-wise intervention on post mastectomy care was taught and practiced by patients from 1st to 7th post operative days regarding wound care, precautions related to operated arm, dietary measures and post mastectomy exercise. On the 7th post operative day i.e., on the day of discharge from hospital post-test one was done and data on self-care practice, arm lymphedema symptoms and arm circumference measurement were collected. Patients were provided with booklet on post mastectomy care. On the 15th post operative day i.e., on the day of first follow-up posttest two was done.