The term "the mentally incompetent" in Korean legal system, refers to persons categorized as "mentally ill person" by Mental Health Act or "person with mental disability" by Act on Welfare of Persons with Disabilities. The mentally incompetent may be restrained of personal liberty due to certain reasons such as involuntary hospitalization or protected by Framework Act on Social Security. The mentally incompetent may also be subject to social exclusion as well as receiving assessment on the degree of his/her risk in causing harm to himself/herself or others. Although psychiatrists are given a relatively high degree of discretion over assessment of mental capacity of the mentally incompetent, doctor`s discretion may also be restricted by patients exercising their rights to self-determination and the practice of informed consent process. The assessment of the mentally incompetent`s mental capacity is used as a basis for making decisions on whether the patient has the capacity to consent to treatment, and whether to allow restricting his/her right to self-determination. The assessment should cover medical standards of the necessity for treatment, the point of assessment, and continuity of the patient`s mental capacity. Informed consent should be secured in each process of medical practice including closed ward admission based on the conclusion of the mental capacity assessment. This dissertation examines the requirements of due process in Mental Health Act with regards to securing the right to self-determination of the mentally incompetent, and Adult Guardianship System in the revised Civil Code. The study then deals with the problems and issues to be taken into consideration in applying the systems mentioned above.