In the area of foreign investment protection, international law can, and sometimes should, draw upon domestic legal concepts and principles derived from comparative law analysis. When properly employed, comparative law may not only help to fill ‘gaps’ in international rules, but may also ameliorate the tension between international law’s claim to universality and the importance of respecting local diversity. Yet, in the history of international investment law, the process of borrowing from domestic laws has been controversial and has been criticised as being biased, arbitrary, and hegemonic. The thesis aims to develop a principled approach to the use of comparative law in international investment law by analysing the legal bases for recourse to comparative law analysis and proposing the proper comparative law methods in the identification, interpretation, and application of international rules. As a first step, the thesis examines the entry points of domestic legal concepts and principles in international investment law based on the rules on sources. In determining the entry points, it highlights the significance of state intention regarding the relationship between international law and domestic laws on particular matters. In particular, this part of the thesis investigates the extent to which the substantive standards and legal concepts in IIAs are aligned with, or diverge from, relevant institutions in domestic laws. As a second step, the thesis suggests the proper comparative law methodology in international investment law. Following the sources doctrine, it develops the appropriate comparative law methods in the interpretation of IIAs based on different legal grounds, with case studies demonstrating the feasibility of the suggested comparative method. Ultimately, the thesis builds a bridge between international investment law and comparative law. This bridge offers a solid legal basis for arbitral decisions, and ensures that the international rules are representative of the diversity of the world community.