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According to the principle of Freedom of contract the parties to the contract can include their preferred condition in the contract. In this way the condition of the best effort to fulfill the obligations is one of the conditions that can be mentioned in the contract, according to which, the obligor will do his best to fulfill the obligation. Such a condition causes the obligor not to think only of relieving himself. Despite the frequent use of such a condition in US contracts, there is no consensus on its validity. In this legal system, Some lawyers do not accept the mentioned condition due to its generality and ambiguity and consider it invalid, and on the other hand, they consider it correct. Recently, in international documents such as Unidroit Principles of International Commercial Contracts and Principle of European Law Service Contract  the condition of the best effort has been recognized and declared valid. In Iranian law, this condition is explicitly accepted in some laws, such as insurance laws and medical law. This article, using a descriptive-analytical method, after explaining the concept of the best effort condition, examines the position of this condition in the legal systems of Iran and the United States, as well as some international legal documents.