The Mongol State at the beginning of the 13th century, founded by Chinghiz-Khan, was a collective body formed by the domains of Chinghiz-Khan himself and of his children. Every individual prince possessed pasture lands and several chiliarchs (chi'en-hu 千戸) and they also owned rights on recently obtained areas as for instance China. This article investigates in which way this system of enfeoffments (fen-feng-chih 分封制) originated by Chinghiz-Khan was continued during the historical course of the Mongol Empire, focusing on the domain of Hulagu, third son of Tolui, a grandson of Chinghiz-Khan, which was located in Mongolia and China. (After the foundation in 1258 by Hulagu of the Il-Khan state in Iran, this domain is for convenience called "the eastern domain".) The eastern domain of Hulagu was composed of a part of the Mongolian yurts and some of the chiliarchs which Tolui inherited from Chinghiz-Khan, of the rights on more than 7000 households belonging to Hulagu Khan in the regions around Ta-tu 大都in China, and on the Ho-tung 河東 and Shen-hsi 陝西 areas, as well as rights regarding 25056 households in Chang-te lu 彰徳路. Furthermore, after the death of Hulagu, the Pao-ch'ing 寳慶 district was added to the rights of the Hulagu-family, after the pacification of the Southern Sung by Qubilai. In 1253 Hulagu, receiving an order from his elder brother Mongke, left for the expedition to Iran. On occasion of the struggle between Qubilai and Ariq Boke following the death of Mongke in 1258, the Mongolian domain disappeared, and he found himself far distant from his still remaining Chinese domain. However, the Hulagu-family had officials which represented their family (the Chung-shu-sheng tuan-shih-kuan 中書省斷事官) participate in the central governmental organizations of the Qubilai-regime, and they further dispatched from Iran two messengers for financial investigations in the Chinese domain. As for the delivery of the income from the domain to Iran, it can only have been possible when Ch'ang-te 常徳 travelled to Iran in 1257 during the Western expedition, and apart from this time, it was not carried out at all. The income was kept in custody by the Yuan-government, and the more than 7000 households of the regions around Ta-tu were returned to the Yuan-government. In this way the eastern domain after the Western expedition, especially the Chinese part, was unable to realise its meaning as the economical basis of the Hulagu family, and on the contrary, one should probably regard the maintenance of the Chinese domain by the Hulagu-family after the foundation of the Qubilai-regime as being carried out to demonstrate the position of the Qubilai government as the suzerain of the Mongol Empire. As for the above-mentioned participation of officials in the Qubilai regime and the dispatchment of envoys by the Hulagu family, it would seem that the accent lies more on the demands of the Qubilai government. This situation of separation between the Chinese domain and the house of Hulagu began to show changes at the beginning of the 14th century. The seventh Il-Khan, Ghazan, opened, as one part of his financial reforms, the sea-trade with China, and he strengthened the friendly relationships with the Yuan-government. Because of this, the formerly returned 7000 households were again offered to the Hulagu house, and also the several kinds of income from the Chinese domain which had been stored up by the Yuan dynasty for a long time, were again sent to the house of Hulagu. This article gives only one example of the historical development of the domain system (fen-feng-chih 分封制) of the Mongol Empire, but I think it's necessary to conduct more studies of this kind in order to understand the significance of the fen-feng-chih system.