BACK GROUND: Our object is to conduct a questionnaire research with psychiatrists who have the experience of mental examination to suggest judgment standard for Not Guilty by Reason of Insanity and Guilty But Mentally Ill(diminished capacity). In addition, this study is expected to reduce the gap between the judgments on Not Guilty by Reason of Insanity and Guilty But Mentally Ill. METHODS: In making judgment on Not Guilty by Reason of Insanity and Guilty But Mentally Ill by experts, the questionnaire research is conducted through e-mail on the judgment standard on each mental disorders. According to the definition of mental disability, the item that shows higher percentage than the items of cognitional ability and volitional ability is considered as the requirement for judging mental disability. RESULTS: In all of 7 major mental disorders and disease groups, the symptoms and characteristics of the disease and the range of the influence of the symptom on the case were higher in 2-4 times than the induced standard (cognitional ability and volitional ability) from the legal and academic definitions of mental disability. CONCLUSIONS: The criteria for judging normal or mental disability is the cognitional ability and volitional ability. The criteria for judging Not Guilty by Reason of Insanity and Guilty But Mentally Ill in mental disability is the symptom and characteristics of the disease and the range of the influence of the symptom on the case.