The Uposatha hall is located at the center of the temple precincts of Theravāda Buddhism. Upasampadā, Pātimokkha reciting ceremony and other most important rites for monks are held there. In this report I would like to present my views on what the Uposatha hall is, through the investigation of the Uposatha hall in Wat Pho. The Chakri dynasty or the Bangkok dynasty became the sponsor of this temple from the days of King Rama I. So the Uposatha hall of Wat Pho was built with the clear intention of Chakri dynasty which may be thought out with the research to follow. King Rama I donated the art works of the epic of the Ramakian, Rāmāyana in Thailand to Wat Pho. Because it is related to the Ramakian that Good defeats Evil in the end of the battle and the story of it honors the king, in which the king compares himself to the God Viṣṇu, who comes to earth in the story as the good king Rama. King Rama I may have wished by virtue of his position as the king to be the reincarnation of God Viṣṇu, coming to earth to solve all the world’s troubles. King Rama I brought many Buddha images from the temples destroyed by the war in cities in the northern part of Thailand and then housed in the halls or the cloisters in the site of the Uposatha hall of Wat Pho. The Lopburi style stūpas at the four corners of the site of the Uposatha hall and the five pagodas behind the cloisters at each corner are built in the reign of King Rama I in which the relics of Shakyamuni Buddha have been enshrined. As for the mural paintings in the Uposatha hall of Wat Pho, King Rama III started to draw them and King Rama IV has completed them. The lives of disciples of the Buddha are depicted in the 29 bays between doors, or between windows and doors there, though the biographical stories and the fixed ten past stories of the Buddha surely appeared on the Uposatha hall in the days of the late Ayutthaya and Tomburi. Here the biographical stories of the disciples are depicted vividly how they entered into the Buddha’s saṅgha. Moreover the daily lives of the lay people are shown clearly. Foreigners on the mural appear friendly. The several scenes of the paintings of no one but Unmaggajātaka telling on Bodhisatta Mahosadha as the Jataka stories, are shown on the upper bays of windows and doors. In his childhood he was very clever. After he grew up, he helped his king on many matters and he dug the tunnel in which he made the palace having automatic doors and so on. Through the story, I have felt the intention of King Rama IV who wanted to make Thai people know about the importance of the wisdom and the modern industrial civilization. The Chakri dynasty wanted to make the people know the importance of Buddhism, Thai cultures, foreign cultures, European civilization and the existence of foreign people are shown in the mural painting of the Uposatha hall of Wat Pho.