Atrial fibrillation is a major risk factor of stroke. Stroke prevention in atrial fibrillation (AF), most commonly with warfarin, requires maintenance of a narrow therapeutic target (INR 2.0 to 3.0). To ensure safe and effective use, it is vital that patients have a sound understanding of warfarin. Therefore, the aim of the study was to determine AF patient's knowledge about using warfarin. Methods descriptive correlational cross sectional research design was conducted in the cardiovascular outpatient clinics of Specialized Medical Hospital at Mansoura University. Sociodemographic data sheet & Warfarin knowledgequestionnaire was designed after an extensive literature review. It contains 24 questions related to warfarin use and international normalized ratio (INR) test. Convenient sample of this study composed of 104 adult patients of both sexes who met inclusion criteria and faceto- face interviews using standard questionnaires were administered to determine the demographic characteristics, medical history and the patient's knowledge of warfarin therapy. Results the result revealed that Patients' warfarin knowledge was poor, with an overall score of 9.9±6.32 (range 0-24). Knowledge deficits were greater in female subjects, older subjects, subjects with less formal education, subjects live in rural, subjects don't work and subjects with long duration of warfarin use. Conclusion There is an increasing need to introduce effective education programs to current patients in order to enhance and update their knowledge with regard to warfarin therapy and INR test.