The problem with regard to the excess of power doctrine, one of the most controversial issues in the procedure of international courts and tribunals, was invoked in certain recent cases, such as the Bosnian Genocide case and the South China Sea case. To solve this problem, it is necessary to examine two relevant legal notions: the principle of competence-competence; and the principle of res judicata. First, the principle of competence-competence has been regarded as a relative power of arbitral tribunals in the sense that it is limited by the application of the rules of treaty interpretation. This means that there are possibilities, in the substantive sense, of an excess of power even when the principle of competence-competence is applied. Second, the international jurisprudence has shown that the force of res judicata can be excluded by a post-award agreement of the parties. Thus, it is possible, substantively and procedurally, to examine the possibility of an alleged excess of power having been committed by an arbitral tribunal and this understanding has been accepted by the ICJ in its jurisprudence. On the contrary, the ICJ has denied the possibility of an excess of power by itself, on the basis of the absolute power of competence-competence and the principle of res judicata. Here, several problems remain unsettled with regard to the difference between the possibility of an excess of power by ad hoc arbitral tribunals and the possibility of an excess of power by the ICJ.