This thesis explores the development of oligarchs and the political institution in Ukraine from 1991 to 2019. In April 2019, Volodymyr Zelensky, a comedian, won a landslide victory in presidential election, revealing the importance of support from oligarch, Ihor Kolomoisky. In a TV series, , Kolomoisky created Zelensky as an upright and ideal president to avenge the incumbent, Petro Poroshenko, for “nationalizing” his Privat Bank, the biggest bank in Ukraine. Based on this battle between oligarch and the former president, main purposes of this research are as follows: The first is to explain network theory, particularly “soviet relation and network”— the connection that people during the USSR relied closely on each other in everyday life, which still existed, and had turned into hidden rules and the “navigation” in modern society that everyone has to obey and follow. The second is to illustrate the formal institution, concerning Ukraine’s semi-presidentialism, including constitution, parliament, cabinet and president — especially the constitution, which has been strategically amended by presidents for their own purposes. The third area is the informal institution, which emphasizes the role of oligarchs in Ukraine, including their main clans in different regions (oblasts). To expand largely the scope of business, they stretched their arms in local administrations, parliament, and government for power and to maneuver the policy either indirectly or directly in their favor. The final is to analyze how formal and informal institutions, through historical institutionalism, influenced each other in each period of presidency and under different constitutional systems. Oligarchs’ manipulation of informal institution makes up for the weakness of formal institution. That is, this thesis intends to prove that oligarchs, from dependence to independence on formal institution, became the “hidden strong hand”, controlling Ukraine’s economy and politics.