Introduction: persuading research activities is the major factor for scientific and technological progress in the societies. The role of universities is essential and a properly prepared scientific dissertation is an important factor for this purpose. This study was carried out to evaluate the major writing principles in dissertations of medical students at Arak University of Medical Sciences.Material and Method: 266 dissertations of medical students graduated from 1373 to 1379 from Arak University of Medical Sciences were studied. In each writing, prepared questionnaires were filled out by academic staffs based on Lickert Format. The data were analyzed by descriptive statistical methods.Results: The highest frequency of dissertations were related to graduate students graduated in 1375 (23%). The highest frequency in academic departments was Department of Interanal Medicine (17.7%) and Department of Ophthalmology (1302%). Ladies were more interested in Departments of Dermatology and Gynocology and men were more interested in Departments of Surgery and Internal Medicine. Considering the kind of dissertations, the most frequency was recorded for observational - cross - sectional studies (44%). No translation or review article dissertation was recorded. 52% of dissertations were evaluated as good, 40% of them were fair and 8% were evaluated as unacceptable. Considering the different parts of a dissertation, "The Abstract" (54%) and "The Results" (52%) were evaluated as the best and the discussion (26.7%) was evaluated as the worst.Conclusion: Although the quality of scientific writing of research dissertations of medical students has improved over the past few years, but preparation of a standard and consistent framework by research deputy of Health Ministry looks necessary for scientific dissertations.