This thesis aims to study the development and institutional change in the Senate of Thailand with two research questions to be answered: 1) How has the Thai Senate evolved? 2) why does the Thai Senate exist? It hypothesises that The evolution and the institutional change in the Thai Senate should be analysed in the broader context of Thailand’s struggling democracy since 1932: 20 constitutions and 13 coups have resulted from a power struggle between the elite and the masses. The power struggle determines the constitution as “the rule of the game”. A constitution designs political systems and institutions. When the constitution changes, it also changes the arrangement of political institutions, including the parliament. The change in the constitution reflects the reality in Thai politics that power relations have changed. The institutional evolution of the Senate is part of constitutional change, which reflects the shift in power relationships in Thai politics. Also, the upper house in Thailand neither represents special interests or social classes nor improves the quality of the legislative outcome. By contrast, the Thai Senate has been “the guardian of the status quo”, a reserved domain for ‘veto powers’ to take certain political domains out of the hands of democratically elected representatives by constitutional means throughout Thailand's political development since the end of absolute monarchy in 1932. Those in power always employ the upper house as a support base to safeguard their power and influence parliamentary politics.

It was found that, first, the pre-1997 Senate 1997 was mainly an appointed House, but the appointee, composition, and powers of the Senate vary according to the Constitution. Except for the 1946 Constitution, which stipulated that the senators shall be indirectly elected, at the initial stage, they were selected by the Senate Selection Organisation consisting of the incumbent MPs. The 1947 Provisional Constitution and the Constitutions of 1949, 1968, 1974, 1978, and 1991 stipulate that the senators shall be appointed. In some eras, the Senate had the power to trust the government and was designed to be the power base.

Second, the Senate under the 1997 Constitution (2000–2006) was fully elected. In addition to reviewing the legislation, the Senate had two significant powers: selecting and appointing persons in independent agencies, which were newly established under this constitution, allowing the public to punish politicians and high-ranking officials. The Senate under this constitution was designed with the expectation of a non-partisan chamber in line with its core aim of political reform. However, according to the findings of this study, the Senate was also a political support base for powerful political parties at that time.

Third, the Senate under the 2007 Constitution was equally split between elected and appointed representatives. On the one hand, it cannot be denied that the design of the Senate under this constitution, drafted after the 2006 coup, must have some level of democratic legitimacy. However, on the other hand, the experience of the Senate under the 1997 constitution demonstrates that if it is entirely elected, the political system would be dominated by political parties with a majority in parliament. Therefore, senators appointed by the nomination committee must include chairmen of various independent agencies and the judiciary. The Senate continues to be responsible for reviewing legislation and has impeachment powers, but appointments to independent bodies are limited. The selection of candidates falls under the authority of the nomination committee, consisting of chairmen of independent agencies and the judiciary. The Senate only has the power to approve or disapprove of elected and appointed senators in the same House. Therefore, a proxy war exists between the majority and the minority.

Fourth, the Senate under the 2017 Constitution is a fully appointed House. This constitution was drafted after the 2014 coup. The transitory provisions stipulate that during the first five years after the general election, senators must be appointed by the junta leader. The Senate has the power to elect the prime minister and members of the House of Representatives. It also has the power to monitor and expedite the government’s implementation of the national strategy and national reforms stipulated in the constitution.

Furthermore, it has the power to vote on draft laws relating to the National Reform program as specified in the constitution and, most importantly, to amend it. However, as well as the requirement for a majority in parliament, the government must receive one-third of the senators’ votes to succeed. After five years under the transitional provisions, the senators shall come from the self-selection of professional bodies, while the power under the provisional legislation will also cease to exist. However, the remaining power under the constitutional amendments requires one-third of the votes from the Senate and the approval of incumbent appointments from independent bodies. This constitutional design reflects a throwback to the pre-1997 Senate.