In the case of “Iran’s Foreign Military Sales” (Case B-1), which encompasses six claims and a counterclaim, proceedings have continued for over four decades. A focal point in the process of filing numerous applications and the rulings issued in this case has consistently been the issue of evidence and the burden of proof. This qualitative research aims to address the fundamental question of the approaches taken by the Iran-United States Claims Tribunal regarding evidence and the burden of proof, as well as the implications of these approaches on the adjudication of Case B-1. The findings indicate that the Tribunal, in each case, has adhered not only to general legal principles - such as ‘actori incumbit onus probandi’ -but also to the unique circumstances and specific conditions of each case, such as the accessibility of evidence, in determining the allocation of the burden of proof. As the parties strive to substantiate the credibility of their evidence before the Tribunal using general principles of international law, which are potentially recognized as applicable law by the Tribunal, they also seek to undermine the credibility of the opposing party’s evidence through various arguments. It is essential for Iran to enhance its precision in referencing the submitted documents and to clarify the technical dimensions, as well as to ensure compliance with the Tribunal’s standards in future rulings, in order to achieve its objectives in other ongoing cases.