Adoption is an institution with a long history that existed in Iran before Islam and was signed by Quran and Sunnah with changes after the advent of Islam. Since the rulings, effects and guarantee of the implementation of adoption laws are determined based on its nature, the discussion of the nature of adoption has an important place in legal institutions. The application and implementation of many legal laws depends on the legal status of adoption. Many opinions have been expressed about the nature of adoption, some consider it to be a unilateral legal act (ایقاع) and some consider it to be just a legal relationship according to a court order.  If adoption is a moral obligation, a  trusteeship a contract or another institution, its rulings and effects will also be different. This research has dealt with this topic with descriptive analytical method and library method. According to the historical background and evidences such The Custom Of Rational Persons , Inexistence of Wellayat over another, etc., which are stated in detail in the article, the conclusion is reached that adoption is a contract and not unilateral legal act or just a court order and ... because it requires mutual will. The commentators of the Qur'an have also considered this institution as a contract. Adoption is a ceremonial contract, based on the interest of the adopted child and is binding, and according to the law on the protection of orphaned children and adolescents, it creates rights and duties for the parties. Among the duties assigned to the guardian are providing future finances, paying living expenses, raising and maintaining the adopted child, and the adopted child is also required to respect and obey his guardian.