The post-World War II development of Japanese party politics was mainly carried out on the basis of the 1955-system (1955-1993) of the Liberal Democratic Party (LDP)’s one-party dominance. Under the 1955-system, Japan’s economy has grown rapidly, but at the same time, it has also caused the emergence of informal institutions such as “Iron Triangle”, “Patron-Clientelism” and “Social Network”, which has led to “Interest-driven Politics (Pork Barrel)” and “LDP Factional Politics” flourished. Therefore, the Japanese political arena started a series of “political reforms” in the early 1990s, making a great impact on LDP’s factional structure. Through the approach of “Historical Institutionalism” and from the perspectives of “changes of election strategies”, “changes of the LDP’s factions’ power” and “changes of political funds” before and after the political reforms, this research aims to discuss the relation between several major political reforms in Japan since the 1990s (mainly focusing on the political reforms of the Hosokawa Cabinet in 1994 and four-time modifications to the Political Funds Control Act from 2000 to 2007) and changes of the long-lasting LDP’s factions by using both past literature and numerical data. 　There are mainly three findings in this research: Firstly, from the perspective of “changes of election strategies”, the LDP chiefly adopts strategies such as “dual candidacy (along with best-loser provisions)” and “forming coalition governments with small political parties” to maximize its number of seats in the National Diet under the new electoral system. The LDP also has an inclination of being more “centralized” and with greater policy-decision dominance. As a result, the capacity of the LDP is greatly increased, which has a strong impact on its factions. Secondly, from the perspective of “changes of the LDP ’s factions’ power”, the link between the LDP’s factions and the candidates or representatives has gradually weakened in the House of Representatives. And in the Cabinet, it is unnecessary for a cabinet member to have a “factional background”. Representatives who belong to no faction (or even non-politicians) still have a chance to take office of important cabinet positions, revealing the fact that the LDP ’s factions have become less important and influential than the past. Lastly, from the perspective of “changes of political funds”, donations are no longer a solid source of income for the LDP, but the income of the LDP’s factions has not decreased but increased between 2014 and 2018. The LDP also has gradually deepened its dependence on “political party subsides”, showing that its ability to raise political funds has been severely curtailed and must rely on new sources of income. However, the influence on the LDP’s factions is not as great as that of the political party as a whole.