Coronary artery bypass surgery (CABC) is a commonly performed surgical procedure for the treatment of blockages in the heart vessels. This surgical intervention aims to improve the blood circulation of the patients' heart muscle. However, after CABC, patients may experience symptoms arising from the effect of the operation. These symptoms include shortness of breath, chest pain, sleep problems and a general feeling of restlessness. These symptoms occur as a result of the operation and can limit patients' daily activities, negatively affect their quality of life, and make the rehabilitation process difficult. Diaphragmatic breathing exercise can be an effective strategy for managing symptoms after CABC. This exercise involves deep and controlled breathing and aims to strengthen respiratory muscles, increase lung capacity and improve respiratory efficiency. Diaphragmatic breathing exercise helps breathing occur more efficiently by reducing the movement of the chest wall and can reduce patients' breathing difficulties. Diaphragmatic breathing exercise provides relaxation in the body by calming the sympathetic nervous system and can reduce stress during the sleep process. In this way, patients can experience a deeper and more restful sleep and feel more energetic when they wake up. At the same time, diaphragmatic breathing exercise improves mental state and reduces the effects of stress by providing focus and mental relaxation. This thesis study aims to investigate the effect of diaphragmatic breathing exercise applied to patients undergoing CABC on symptom severity, sleep quality and anxiety. The importance of this study is that it has great potential for managing symptoms after CABC and improving patients' quality of life. Diaphragmatic breathing exercise is expected to be effective in reducing symptoms after CABC, allowing patients to improve their respiratory functions and general health status. Additionally, investigating the effects of this exercise on patients' sleep quality and anxiety may help nurses adopt a more holistic approach in healthcare delivery.