Based on the theory of conversion; if an invalid legal act contains the elements of another valid legal act, in the event that the virtual will of the party is verified by the judge, the invalid legal act will be transformed into a valid legal act. The purpose of the upcoming Thesis is to find ways to identify it in Islamic jurisprudence and Iranian law, and to evaluate the nature of the theory of conversion and to analyze its foundations in Civil law. To achieve these goals, descriptive-analytical approach and library method have been used. According to the findings of this research, in contemporaneous legal doctrine, the originality of the concept of conversion means that the this institution, despite its similarity with some legal institutions, has a distinct and independent concept and responds to objective, moral, social and even economic considerations.

The findings of this Thesis show that there are many Items similar to the theory of conversion in Islamic jurisprudence texts. These cases are mostly has been studied under the title of condition contrary to the requirements of the contract and the conflict between the objective will and the subjective will. Based on the analysis that Imamiyyah jurists have presented in relation to some jurisprudential examples such as sale without price and rent without payment, the words "Ijab'' and "Qubul'' are used in a sense that there is no ambiguity in them, but there are evidences that cause Let the words be used in a meaning other than the customary meaning, so if in the sales contract the seller says in his request, I sold this product to you without a price, and the customer also accepts and bills for it. The contract is recognized as a gift due to the existence of verbal evidence or when they are manifestations of the objective will. In Islamic jurisprudence, can be called the conversion is essentially different from what is proposed in western law, because firstly, the conversion is based on a hypothetical will, and secondly, the determination of this will is the authority of the judge, Therefore, he has an active role in this regard This research shows that in civil law, including Iranian laws, the theory of conversion has been accepted in some cases. Applying of conversion by the judge is compatible with important legal principles such as Pacta sunt servanda, legal security and the authority of the judge In Iranian law, conversion through partial annulment, useful interpretation, and description of legal practice are examples of this coordination with the authority of the judge. Therefore, the acceptance of this theory in Iranian law does not face any obstacle.