This paper explores the profound impact of regional economic integration on international commercial law by analyzing the cases of the European Union (EU) and the Association of Southeast Asian Nations (ASEAN). The study first outlines the theoretical foundations and main types of regional economic integration. It then delves into the high degree of legal unification in the EU and the flexible legal coordination in ASEAN. By comparing their legal systems, corporate law, contract law, and dispute resolution mechanisms, the paper reveals how the EU plays a leading role in international commercial law through its strict legal framework, while ASEAN achieves regional cooperation through non-binding consensus mechanisms. The research finds that regional economic integration not only enhances legal standards and innovation in international commercial law but also significantly impacts the legal compliance and business operations of multinational enterprises. The paper concludes with policy recommendations to strengthen regional legal cooperation and further harmonize international commercial law, and it suggests future research directions, including the impact of other regional economic integration organizations and deeper studies into specific legal fields.