Patients diagnosed with acute coronary syndrome have found that deep breathing exercises effectively reduce physiological parameters. The study aimed to explore the effect of deep breathing exercise application on physiological parameters among patients with acute coronary syndrome. Design: To achieve the aim of the current study quasi-experimental pre-test- post-test with a control group design was adopted in this study. Setting: The study was conducted at the coronary care unit, cardiology ward, and cardiology clinics at Sohag University Hospital. Sampling: A total of 100 with acute coronary syndrome were selected from previous settings based on purposive sampling Then divided randomly into two groups: the experimental group and the control group in each one (n=50). Tools for data collection: Tool I: Structured questionnaire; composed of two parts. Part 1: Patients' demographic data. Part 2: Patients' Medical History Assessment Sheet, Tool II: Physiological Data Assessment Sheet.Results: There was a statistically significant improvement in physiological parameters such as heart rate, respiratory rate,  blood pressure, and oxygen saturation among patients in the experimental group compared to the control group.Conclusion: According to the results of the study, the deep breathing exercise application has a significant positive effect on improving physiological parameters among patients with acute coronary syndrome. Recommendation: Breathing exercises are suggested to be applied to routine care given for patients with acute coronary syndrome.