Background: Fatigue is common after major surgery such as cardiothoracic surgery where its technique involves sternotomy incision and postoperative mechanical ventilation which may affect the breathing ability and may delay recovery. Fatigue is generally defined as a feeling of lack of energy and motivation that can be physical, mental, or both. Breathing exercises enrich and support the circulation and blood flow all over the body by improving the quality of breathing. Respiratory efficiency addresses the physical and psychological conditions of the patients. The aim of the study: was to evaluate the effect of breathing exercises on post-operative fatigue severity after cardiothoracic surgery. Subjects and methods: A quasi-experimental study was used to fulfill the aim of this study. This study was conducted at the cardiovascular ICU and cardiovascular inpatient units in International Medical Center. A sample of 50 adults undergone cardiothoracic surgeries was included in the study. The tools of this study consisted of four parts; Patient demographic characteristics and clinical data sheet and the Rhoten Visual Analogue (VAFS) Fatigue Scale. Lung capacity was measured using incentive spirometry.  The results: revealed that fatigue severity was lower during performing breathing exercises than before performing breathing exercises and was lower after a week of performing breathing exercises.  Recommendations: The study recommended discussing with the responsible health team the availability of setting breathing exercises as a part of routine pre and post-operative care.