Internationally mandatory rules are rules relating to public policy which should be applied directly to certain international situations, regardless of the applicable law designated by the conflict of laws. For the first time in the history of Chinese private international law, the PRC passed the Act of the PRC on Application of Law to Foreign-Related Civil Relations, which defines the direct application of the internationally mandatory rules in Article 4. First, this essay introduces the basic legislation regarding the internationally mandatory rules of the PRC. Second, this article provides criteria to define internationally mandatory rules. Third, this essay explores the specific contract fields of the internationally mandatory rules in the judicial practice of the PRC. Finally, this article analyses how the Chinese internationally mandatory rules were extraterritorially recognized by the courts of the State of New York of the United States and Hong Kong Special Administrative Region.