Medical expert’s opinion about psychological situation and extent of responsibility is known as a reason which to juridical courts refers for sentencing of incapacity. It is current in many countries. In Iran in the most situations medical expert opinions based on being insanity or non-intellectual growth are determinant, so expert opinions are very important. Here there is a question: contrast ability of expert opinion (as a reason) in rejecting other opposite evidences included Witnesses testify, local researches or person history involves social and legal practices? In this article we’ve attempted response this question from point of legal view. The result is amount of expert opinions authority about qualifying insanity is subjected for the nature of insanity and its connection with medicine and cannot be overlooked apart from the best expert opinions and isn’t dismantled, whoever there are conflicted reasons. But about non-intellectual growth, one can refer to it because of its nature and definition and its connection with medicine and cannot resist in contrast with religious and legal reasons and we can overlook it. About recognize immaturity (as the third reason of incapacity) regarded to legal standard that is not reached to certain age, we can consider expert opinions as a reason like determination of intellectual growth.