This article examines the power relationship between the prime minister and ruling party legislators. I theoretically explore the power relationship between the prime minister and ruling party legislators, and examine legislators' parliamentary voting, focusing on the political process of postal service privatization of 2005. This analysis presents three arguments. First, theoretically, when the prime minister attempts to achieve a project, risking his or her job, he or she can firmly control ruling party legislators. Second, empirically, the anti-Koizumi legislators' rebellion was based on their unfounded assumption about the prime minister's behavior or ultimate outcomes. Third, the anti-Koizumi legislators' electoral incentives to secure their reelection and ruling incentives to weaken or overthrow the Koizumi Cabinet drove them to rebel against the prime minister in the Lower House, while the ruling incentives alone caused the rebellion in the Upper House.I would like to thank Michael D. Folland, Hideki Kido, Jung A Kim, Akitaka Matsuo, Etsuhira Nakamura, Robert Pekkanen, Jan Reinsch, and three anonymous referees far their helpful comments on this paper. 233 the postal service privatization act of 2005 235reforms, such as reducing the size of government and cutting government expenditures. Also, he further reinforced party leadership by decreasing the influence of factions or zoku legislators. This article has three purposes. First, I discuss the structural power relationship between the prime minister and ruling party legislators. The Japanese prime minister has strong formal authority. In particular, the prime minister can dissolve the Lower House and, as the president of the ruling party, expel hostile party legislators on his or her own. Yet, earlier prime ministers have not exercised this right. Koizumi was the first prime minister to attempt to wield this authority. At the same time, under the parliamentary cabinet system, the Cabinet is built upon the confidence of the Diet. The Cabinet is dependent on support from a ruling party (or ruling parties) holding a majority of seats in the Diet. Therefore, the prime minister needs support from ruling party legislators in order to maintain his or her government and pass bills. When the prime minister and ruling party legislators conflict with one another, how do both sides attempt to control each other? What outcomes does their conflict eventually cause? By analyzing the power relationship theoretically and building its model, I aim to establish a general theory on the power relationship between the prime minister and ruling party legislators. Second, I examine the expectation and strategy of both Prime Minister Koizumi and the LDP legislators in the process. What result did both sides expect and desire? How did both sides try to control each other? How did they predict opponent strategy and behavior? Third, I explore the motives of the legislators who voted against or abstained on the postal privatization bills. I can point out several possible reasons that ...