One of the important issues in the law of contract is the impact of third-parties cooperation with the contractual promisor in breach of his obligations. In legal systems of France and Belgium, the third-party cooperation in breach of contractual obligations, has been considered to be the cause of civil liability. This research with an analytical, logical and comparative method, meanwhile explaining the Basics, conditions and effects of aforesaid liability in legal systems of France and Belgium, has investigated the viewpoint of In legal system of Iran. Based on the results of this research, in French doctrine and precedent, joint and several liability of the promisor and the third-party is the theory accepted for the compensation of the damages caused to the promise. Moreover, in the case of contracts that the third-party concludes with the promisor in order to breach his obligations, non-opposability or annulment of aforesaid contracts has been propounded as the appropriate way to restore the previous situation. In legal system of Iran unenforceability of aforementioned contracts is acceptable. It is also defensible to suppose the promisor and the third-party equally liable toward the contractual promise for the damages resulting, unless their conduct efficacy is different.