The International Institute for the Unification of Private Law (UNIDROIT) established UNIDROIT principles, which could be applicable as international unified rules. The UNIDROIT Principles plays the role of interpreting and complementing CISG and functions as a law applicable to international commercial disputes. As shown by cases of practical application so far, the principles are expected to be applied frequently to international commercial arbitration in the future. In the situation that there is no internationally unified judicature, it is necessary to promote rational dispute resolution and legal stability through arbitration by adopting the UNIDROIT Principles of Lex Mercatoria as a governing law of international commercial contracts. In conclusion, UNIDROIT principles, along with CISG, are expected to playa great role as the applicable law of international commercial contracts and as standards for resolving international commercial disputes.