Colonialism is the policy or method whereby one state or race governs and dominates other states or races. In the process of developing international law, colonialism is involved with the domination of non-European states by European states in the first half of the twentieth century. Since colonialism was a policy of European nations, they had special rights by which they could participate in the process of making international law; today, their orders gradually developed into a universal regime of international law. The main issue is that the contemporary international law of this universal regime derives from European empires and reflects hierarchic and Eurocentric thinking. This raises the question of whether current international law has been formed only by those European nations that have policies of colonialism. The question could be answered by analyzing the mandate system under Article 22 of the Covenant of the League of Nations. At first, the regime was designed to address the matters of the colonies and their native peoples and was organized with the legitimacy of international law. However, the Mandatory Powers utilized the mandate system to secure their control over colonial areas rather than organize them, as United States President Wilson intended. First, this paper examines the matter of the conflict between the general principle of the mandate system and Article 22, paragraph 6 of the Covenant of the League of Nations which regulates the territories of “C” mandate. Second, the reality of mandate system was that the mandatory powers of the “C” mandate committed economic and cultural exploitation under the regime similar to colonial policies. This was demonstrated by exploration of the phosphate exploitation in the Mandate over Nauru Island, the Apartheid perpetrated by South Africa against South-West Africa and the mandate system of Japan. As a result, the powers of the day utilized the regime as a method of bolstering their colonial holdings, and these colonial features were particularly apparent in “C” mandates. Indeed, by analyzing how the regime was regulated by international law and examining the two general principles from the system, the international law of the day can be traced. Since the establishment of the mandate system was about one hundred years ago, it seems that it was not relevant to the modern international law. However, as modern international law has been developed from the early twentieth century, identifying the system under international law at the time can be helpful for understanding it.