Background: The Cardiac Electrophysiology Study (EPS) procedures involve both diagnostic tests and interventional treatment procedures. Therefore, to provide proper care, Critical care nurses must be knowledgeable and competent in both the technical and psychosocial aspects of an electrophysiological Study. Aim of the study: To evaluate the effect of instructional guidelines on critical care nurses' knowledge and practices regarding cardiac electrophysiology. Research Design: A quasi-experimental design was recruited to conduct this study (pre/post-test). Setting: This study was carried out at a cardiac care unit affiliated with Sohag University Hospital. Subjects: A convenient sample of 50 critical care nurses working in the selected cardiac care unit. Tools: Two tools were used Tool 1: Structured interviewing questionnaire: developed by the researchers and divided into two parts; part 1: Nurses' personal data sheet, part 2: Nurses' knowledge assessment questionnaire, and tool II: Nurses' practices concerning patient care before, during, and after electrophysiology. Results: The results revealed that there was a positive significant correlation (P<0.001) between nurses' knowledge and practice mean scores pre and post-instructional guidelines implementation. There were highly significant improvements in nurses' knowledge and practices post-instructional guidelines implementation (P<0.005). Conclusion: The instructional guidelines implementation had a significant positive effect on improving nurses' knowledge and practices regarding cardiac electrophysiology. Recommendations: Develop and implement a nursing educational program for critical care nurses to improve their level of knowledge and practice regarding cardiac electrophysiology.