Nowadays, the International commercial transactions have become a regular activity of many businesses in today's globalized economy, and that’s why, with the increasing complexity of international trade, the regulation of such transactions has become more and more important. The traditional sources of international law, such as treaties and conventions, have become inadequate, in some cases, to satisfy the needs of international commercial transactions. As a result of this, a body of international commercial laws known as the lex mercatoria (hereafter, LM) has emerged as another source of law in national legal systems, whose relationship with the national system I will analyze in brief. We can define the LM as a collection of customs, practices, and rules that have evolved through international trade and have gained recognition as a source of law for international commercial transactions. The LM, is characterized by not being codified in any single document, being the result of the principles and practices that have been developed through the practical application of international commercial law. 1 The applicability of the LM has been the subject of ongoing debate among scholars, with some arguing for its use as a tool to regulate international trade, while others have been more critical of its role in international law. In this study, I will focus specifically on the applicability of the lex mercatoria in national legislation so, the primary objective of this study is to examine if the LM has been integrated into national legal systems and how it has been integrated