The purpose of this study is to identify how Mercosur works in the political and economical Brazilian international insert. The hypothesis to be verified defends the importance of the International Organizations, especially Mercosur, as an instrument of power capable to maximize the gains of the Brazilian state in an anarchical system while it guarantees the transparency and the increment of the flows of information among the states, reducing consequently the costs in the international negotiations; it foments the commercial flows intra-block reducing the commercial tariffs among their members; it contributes to the narrowing of the relationships with the main Brazilian commercial partner, the European Union; it checks credibility to the positions adopted by Brazil in the multilateral forums as the World Trade Organization; and it impels the Brazilian insert as an instrument of defense of their national interests.