This thesis investigates the function of the Oikoi building at Labraunda in the 4th century BC. The Oikoi building was constructed as part of the extensive Hekatomnid building program at Labraunda in the 4th century BC with an unusual ground plan. With its two rooms and a common tetrastyle in antis porch, the Oikoi building is an excellent example of experimental Hekatomnid architecture. Yet, because of the uniqueness of the building, the question of its function has remained unanswered since it was first excavated in 1948. In addition, the continuous use of the building throughout the centuries and its reconstruction in the 6th century AD caused the loss of the archaeological material of the 4th century BC. In order to find the function, two strategies have been used. First, a catalog of buildings with similar ground plans has been provided in this thesis to see if the ground plan fits a specific function. However, the comparative study showed that buildings with two rooms might have several functions, such as a temple, a banqueting hall, a prytaneion, and an ekklesiasterion. The possibilities of the Oikoi building being a temple, a banqueting hall, or an ekklesiasterion were ruled out due to the Oikoi building’s architectural features, its place in the sanctuary, and lack of sufficient evidence. Second, epigraphic research was done to see if the name “oikoi” given to the building on the architrave inscription provides information on the function of the building. By using Hellmann’s (1992) study on the inscriptions from Delos, possibilities such as the Oikoi building being a treasury or a storage unit were examined. This thesis stands in a grey area on that function since there is no architectural or epigraphic evidence to eliminate this possibility. When we turn to the building itself, the context of the inscriptions found in the building showed that the Oikoi building functioned in relation to the office of the shrine, and there was a cult of Hestia related to the Oikoi building at least in the 2nd century AD. If the building was used continuously and the function did not change in the 2nd century AD, the Oikoi building being some kind of unusual prytaneion is possible also considering the Hekatomnid ideology of the 4th century BC.