One of the principles governing the sale contract as the most important exchange contract is the "principle of balance between the parties to the contract". Based on the theory of balance in the sale contract, none of the parties to this contract should be placed in an unequal and unbalanced situation compared to the other, because otherwise, the principles of contractual justice and fairness will be at risk, and the contract is not only not a vehicle for balance and relative equilibrium between its parties, but on the contrary, it is a tool for imposing the superior will of one party on the weaker will of the other party. Accordingly, the sale contract is the most important exchange contract and is based on the relative balance of the parties and any factor that somehow disrupts this balance is considered contrary to the nature of this contract. In French and English law, the balance between the parties to the sale contract has been the focus of jurists. However, the question is whether the aforementioned issue has also been the focus of jurists in Iranian law? In this article, its concept, manifestations and foundations in English and French law, which have identified this theory for the first time, and in Iranian law, have been examined. Also, the manifestations of the acceptance of this theory in the judicial decisions issued in these three countries have been studied. The main conclusion of this article is that the acceptance of the theory of balance between the parties to a contract of sale is to ensure greater exchange justice between them and to prevent possible abuses by the stronger party to the contract against the weaker party in the contract of sale in French، English and Iranian law.