This paper studies “obscenity of purpose violation” as a jurisprudential principle. The most important presuppositions and results as well as the jurisprudential background and the scope of applicability of the issue among the jurists have been also covered. The permission and legitimization of such rules such as using legal devices to avoid usury requires an in-depth research work into the possibility of legitimization of rules going counter to the other goals of this rule. The search into the first sources of jurisprudence and the works authored by the ulema in this field, as well as the study of single elements and determining the rule’s presuppositions have constituted the backbone of this study. The existence of examples of the commitment of the past jurists on the obscenity of purpose violation, especially Imam Khomeini (who has resorted to this principle to support the prohibition of applying legal devices to avoid usury), tell about the wide acceptability of this principle irrespective of its limited use in the process of reasoning. Also the belief in the existence of rational decency and indecency, purposefulness of the Almighty God in legitimization of the religious orders, distinctiveness of purposes and the assurance over their violation are among the cases obscenity of purpose violation needs to be functional.