Introduction: The importance of the periodontal health maintenance and promotion in diabetes mellitus has been supported. Evidence showed that many patients are unaware about effects of diabetes mellitus on oral health. The aim of this study is to evaluate knowledge, attitudes and practice of diabetic patients concerning the risk of periodontal disease and prevention. Materials and Methods: In this cross sectional study, 156 diabetic patients referring to Yazd Diabetic Research Center were recruited by random selection. Patients completed questionnaires, which included 29 questions about their knowledge, attitude and practice of oral health in diabetes mellitus. The data was scored and analyzed. Result: The results showed knowledge (58.64) and attitude (46.86) scores of diabetic patients were moderate, while their practice (24.3) score were poor. There is statistical significant relation between mean scores of knowledge, attitude and practice of patients with their educational level and age. Conclusion: This study showed knowledge, attitude and practice level of diabetic patients were insufficient on oral health care.