Sometimes, the parties of a contract for any reason do not want or cannot agree about some terms of the contract such as the quantity or quality of the object, the price, the method of the payment, duration of contract at the time of conclusion of the contract. Therefore, they conclude the contract and postpone the determination of these terms or only rely on setting up a criterion to determine it. There are disagreements over the validity of this type of contract that is refferedto as open contract. According to the traditional doctrine, one of the basic principles of validity of a contract is the need to resolving of ambiguity of contract and if a deal has vague and open provisions, it would be void for reasons such as lack of sufficient agreement, absence of intention, the impossibility of completing the contract, the chance of gain or loss in a contract and conflicting with reasonable men practices and public policy. But the analysis of these arguments shows that none of them are perfect and sound reasons for invalidity of this kind of contract and there are logical answers to them. Moreover, common interest, the necessity of maintenance of legal relations and their stability, the needs and requirements of commerce and economic efficiency require that this kind of contract must not be void when some of its elements and conditions are open whereas the ambiguity of the contract is not such as to lead to a dispute.