Purpose: This study is to evaluate the effect of abdominal breathing on postoperative pain, state anxiety, blood pressure, and heart rate of cardiac surgery patients treated in intensive care unit. Methods: A non-synchronized design of nonequivalent control group pre-post test was used. Participants were divided into an experimental group(n=22) and a control group(n=21). Just usual postoperative treatments were provided to the control group, while abdominal breathing interventions were provided to the experimental group, in addition to usual treatments. The intervention was consisted of 4 stages-introductions, breathing perception, breathing training, and closing-and it lasted twelve minutes in total. Pain, state anxiety, blood pressure and heart rate were the dependent variables of the abdominal breathing. Results: Those who carried out abdominal breathing showed a significant reduction in pain, as well as decrease of systolic blood pressure. However, it turned out to have no effect on state anxiety, diastolic blood pressure and heart rate. Conclusion: Abdominal breathing is simple and harmless and is effective for reducing postoperative pain.