Purpose: The purpose of this study is to investigate the trends and traits of moral sensitivity and reasoning over six years of medical school. Methods: The participants in this study consisted of 217 first to sixth-year medical students of Sungkyunkwan University School of Medicine in Korea. Essay type questionnaire was used for evaluating moral sensitivity and the Korean-Defining Issues Test (KDIT) was used for evaluating moral reasoning. Results: It was found that the medical students’ sensitivity of ‘public welfare’, ‘prospective result’ and ‘role-taking as a doctor’ was significantly higher than that of the pre-medical students’. Moral sensitivity increased in the 3rd and the 4th years but decreased in the 5th and the 6th years. Moral reasoning decreased in the 3rd and the 4th years but increased in the 5th and the 6th years. No correlation was observed between moral sensitivity and reasoning (r=0.033). Developmental trends of moral sensitivity and reasoning showed a significant difference in terms of year of medical school. Conclusion: The results suggest that medical school curriculum tends to impct students moral sensitivity. This school has implemented the problem-based learning curriculum into the 3rd and the 4th years. It is necessary to consider the developmental state of the student’s morality when developing an effective medical ethics program.