Offer is created by offeror`s intention. There are three opinions about the revocation of offer. The First one is that the offer is always revocable and any agreement as to the contrary is void. The supporters of this opinion are divided into two groups. Some jurisprudent and lawyers believe that offeror who revoke of offer is doing his right and have no duty to compensate. But another group believes that the revocation of offer is a kind of tort and offeror is responsible. The second one is that, revocation is not possible. Among Shiite jurisprudent, Tabatabaei Yazdi (writer of Orvah) accepted this opinion accordint to four sources of Imamie but rejected it because it is against what accepted by all Shiite jurisprudent (common belief), i.e. revocation is always free at any time for oferror. I believe this opinion (offer is not revocable) can be accepted in Shiite jurisprudence and is not rejected by all Shiite jurisprudent. The third opinion is that the offeror can revoke its offer but in some conditions, it is not possible for him to revoke it. For example, when it indicates, whether by stating a fixed time for acceptance or otherwise, that it is irrevocable, or if it was reasonable for the offeree to rely on the offer as being irrevocable and the offeree has acted in reliance on the offer. CISG accepted this idea. All these three opinions, based on their own philosophy. The 2nd opinion which is accepted in this article is based on Unilateral Obligation Theory. This theory is accepted by some great jurisprudent and I focused on this theory to stablish the idea of irrevocability of offer.