All medical researches concentrate primarily on advances of medical knowledge and strategies for prevention, diagnosis and treatment of diseases. However, since most of the medical researches are being carried out on human subjects, they often pose ethical problems. Clinical investigations from which the human subjects involved may not derive any personal benefits include studies of physiological function in health or disease, investigations of new drugs, and the trial of new procedures. This paper discusses the ethical problems associated with medical researches, especially clinical investigations involving human subjects directly or indirectly. One general principle that has been wildly accepted is that patients or volunteers involved in the clinical investigations should be fully informed about the experiment in which they participate. However, ethical problems in those clinical investigations should be evaluated by the three major biomedical ethical principles, or the principle of autonomy, the principle of beneficence, and the principle of justice. This paper also presents the 'Helsinki Declaration' advocated by the World Medical Association in 1964 and revised thereafter several times as a practical guideline for the ethics of medical research.