Abstract：In 2000, after the presidential election, the PRI lost power to the opposition party, which serves a landmark representing Mexico's completion of the transition from one-party rule to democracy. The transition has the following four characteristics. First, it was led by the PRI instead of any opposition parties; second, it was motivated by a series of election reforms instead of civil unrest; third, it was based on gradualism and a multi-phased development instead of any shock therapy; fourth, it was very smooth and peaceful instead of major political instability. As a result of political transition, the PRI achieved a self-transition from the official party to a parliamentary democracy party. Especially after 2000, it made a sustained effort to adapt itself to fast-changing political realities. Its guiding principle was transformed from revolutionary nationalism to democratic socialism. As a result of growing significance of governors, it constructed a new pattern of power allowing them to share power once monopolized by the central leadership. To promote its popularity, it attempted to make itself less elitist and took measures to enhance its influence over the lower class. In addition, it continued to strengthen its primary election institution to encourage inner-party democracy. The PRI's experience shows that a once-ruling party like it has the ability to survive as an opposition party in a competitive party system. The PRI remained to be one of the most important political parties in Mexico even after it lost ruling power in 2000. It is especially noteworthy that the party showed remarkable determination and intelligence to achieve a full rejuvenation of it. The above-mentioned facts show that the PRI is one of major beneficiaries of the political transition in Mexico that was initiated actually by the party itself.