Diabetes Mellitus (DM) knowledge of various aspects is critical for disease prevention, management, and control. Several studies, however, have consistently shown that the general population is unaware of diabetes. Objective: To assess Knowledge about various aspects of diabetes among known diabetic patients attending diabetic clinic at Pakistan Institute of Medical Sciences (PIMS) Hospital, Islamabad. Methods: During this descriptive cross-sectional study data were collected from 200 patients through a self-structured questionnaire. patient was questioned about their demographic information as well as their personal and diabetes-related history. Participants were asked to respond to each question in the questionnaire using their knowledge and understanding. Data were obtained in frequency and percentage to examine sociodemographic variables. Results: In the current study, more females, 108 (54 %) contributed in the study as opposed to males, 92 (46 %). Results showed that 170 (85%) of diabetic patients had type 2 diabetes, 24 (12.0 %) were suffering from type 1 diabetes, and 6 (3.0 %) had Gestational diabetes mellitus GDM.  Conclusions: There was considerable knowledge about diabetes in known diabetic patients but still there is a dire need to increase knowledge and awareness regarding disease understandings, causes, complications, management, and its preventive measures.