To identify the subject of this Article, the pure defense first must be separated from hybrid counterclaim. Hybrid counter claim has a dual function: the first aim of this claim is to reject the adversary claim. On the other hand, as similar as the pure counterclaim, it renders a claim, out of the object of principal claim. In Iranian Law, some proving criteria have been rendered by legal writers to distinguish these two judicial acts. In these theories the probative value of the defendant’s documents, determine the necessity of filing a counterclaim. Upon analysis of this paper, the proving criteria in theory and practice are not convincing for distinction of counterclaim from merit defense. Criterion accepted in this Article is that in counterclaim, the defendant claim to get a profit other than mere reject of adversary’s claim. In spite of apparent simplicity of this criterion of distinction of counterclaim from merit defense, its practical application is not without complexity.