Although definitions of legal insanity differ from state to state, generally a person is considered insane and is not responsible for criminal conduct if, at the time of the offense, as a result of a severe mental disease or defect, he was unable to appreciate the nature and quality or the wrongfulness of his acts. Insanity defense is an important legal matter that criminal acts by psychopaths are related with the most of violent crime. Furthermore, according to the result of insanity defense one can be sentenced to death in South Korea. The Korean Criminal Law has the characteristics of Continental law. Nevertheless, the Korean Criminal Law cannot create the clear and concrete version unlike German Criminal Law that we could apply to the cases on insanity defence. Most countries have tried to provide rational provisions to identify insanity defence. The effort for the better direction about insanity defense and evaluate test instrument has been shown in England and United States` history. The history of the insanity defense in modern times comes from the 1843 case of Daniel M`Naghten, who tried to assassinate the prime minister of Britain and was found not guilty because he was insane at the time. The public outrage after his acquittal prompted the creation of a strict definition of legal insanity which is known as the M`Naghten Rule. In the Durham case in 1950s, the court ruled that a person was legally insane if he "would not have committed the criminal act but for the existence of a mental disease or defect." The Model Penal Code, published by the American Law Institute, provided a standard for legal insanity that was a compromise between the strict M`Naghten Rule and the lenient Durham ruling. Those of historical legal experience in U.K. and U.S. teach to Korean criminal justice system what is rational provisions to identify insanity defense. It is time to reform the Korean Criminal Law, article 10, to be clear on criminal insanity.