This dissertation aims to examine the institutional transitions of the Inner Asian groups in the Central Plain during the Sixteen Kingdoms period and Northern Dynasties. Starting with an examination on the origin and development of Sinicization theory in the West and China, the first major chapter of this dissertation argues the Sinicization theory evolves in the intellectual history of modern times. This chapter, in one hand, offers a different explanation on the origin of the Sinicization theory in both China and the West, and their relationships. In the other hand, it incorporates Sinicization theory into the construction of the historical narrative of Chinese Nationality, and argues the theorization of Sinicization attempted by several scholars in the second half of 20th Century. The second and third major chapters build two case studies regarding the transition of the central and local institutions of the Inner Asian polities in the Central Plain, which are the succession system and the local administrative system. In the first case study, through applying the crown prince system, the Inner Asian rulers reached the centralization of authority, which was different from and even more centralized than the Han tradition. In the second case study, the polities of the Sixteen Kingdoms Period and Northern Dynasties largely followed the Inner Asian political tradition and the Inner Asian groups also remained as units inside the polities. The two case studies show the transition of the institutions of the Inner Asian polity in the Central Plain. The transition is neither a one-way change from Inner Asian institutions to Han and Jin institutions nor a simple hybridity. For different institutions, here the succession system in the central government and the administrative system in the local level, the dynamics for the transition are also not the same. This dissertation approaches the Chinese history with articulating not only what these Inner Asian groups took from the Chinese tradition, but also what they contributed to the institutional changes in Chinese history, which reshapes our understanding of what we call “Chinese” institutions, in other words, Chineseness.