The general studies concerning the International Organizations and their Member States often elude the subject of the relationship between the former to Conventions concluded by the latter. International Organizations as Third Parties under the Law of Treaties is a complex issue; it is indeed difficult to consider an International Organization as a perfect stranger to Member States Agreements. In this context, the European Union represents a standard example of an Organization developing a strong form of integration for its Members. The analysis of the European Union as a Third Party to Conventions between Member States reveals the lack of uniformity of this status. In the framework of the International law, the European Union can be identified as a Third Party in a multitude of cases depending on the variability of the objectives and the competence of the Organization and the application field of the conventions of its Members. Moreover, in the framework of its legal order, the Organization acts in various forms as regards to Member States Agreements. In this way, the relationship between the European Union Law and the Conventions of the Member States is developing either under the sign of the conflict of norms or the conciliation of norms.