This study analyzes the evolutionary trajectory of the concept of the ‘principle of equality’ in international investment arbitration as a procedural principle, tracing its journey from its historical origins in the Magna Carta (1215) to its contemporary manifestations in foundational instruments such as the New York Convention (1958), the UNCITRAL Model Law on International Commercial Arbitration (1985), the ICSID Arbitration Rules, and international arbitral practice. The role and status of the principle of equality are examined, with an emphasis on its inextricable link to principles such as ‘fairness,’ which serve as guarantors of the legitimacy of the international arbitral system. The findings of this research indicate that while Article 18 of the UNCITRAL Model Law, by stipulating “equal treatment,” has been dubbed the “Magna Carta of arbitral procedure,” the ambiguity in the operational definition of this principle and the specifics of its interaction with other governing rules in this field have created significant interpretive challenges. Indeed, the provisions of arbitration treaties, institutional rules, and national laws uniformly mandate, whether explicitly or implicitly, the obligation to treat the parties equally throughout the arbitral proceedings. Nevertheless, the precise contours of procedural equality remain ambiguous. Through an analysis of leading international investment arbitration awards and the rules of prominent arbitral institutions, this study demonstrates that the realization of “justice” in this field hinges on a transition from “formal equality” towards “substantive fairness based on the facts of each case.”