The Senate firstly originated in Thailand in the form of Council of State and Privy Council in the reign of King Chulalongkorn (Rama V). Till the reign of King Prajadhipok (Rama VII), the mentioned Privy Council was designed to exercise in parliamentary procedure as parliamentary model for preparing and promoting the understanding of democratic governmental system.
In 1932, the form of government of Thailand was changed from absolute monarchy to the democratic regime of government with the King as Head of State. The Constitution of the Kingdom of Siam, B.E. 2475 (1932), the first Constitution of Thailand, designated the first Thai National Assembly as a unicameral parliament, namely the House of Representatives that being composed of members of two categories: firstly, elected by the people and secondly, appointed by the King. Both groups are equal number.