Aim of study: was to identify the effect of applying shoulder exercises on shoulder function after modified radical mastectomy. Research design: Quasi-experimental design was utilized. Setting: data were collected from the surgical oncology department and surgical outpatient clinic at South Egypt Cancer Institute - Assiut University. Subjects: The study was conducted on (80) adult female patients with breast cancer scheduled for modified radical mastectomy and were randomly and alternatively divided equally into study and control groups; (40) for each. Tools: data were collected using three tools: Tool (I): Structured Interview Questionnaire; including two parts, tool (II): Shoulder range of motion assessment questionnaire and tool (III): Shoulder functional ability assessment questionnaire. Results: More than half of women in both groups, their ages ranged between (40 - 60) years with mean ages (45.5±11.7) for the study group & (44.8±10.8) for the control group. Majority of the study group had normal shoulder function while most of the control group had difficulty in shoulder function (80% & 60%) respectively one month after exercise. Conclusion: Utilizing post mastectomy shoulder exercises significantly improves shoulder function and shoulder range of motion for patients undergoing modified radical mastectomy. Recommendations: Increase level of awareness among patients regarding shoulder exercises