This research investigates the heat transfer characteristics of two-phase closed thermosyphon on various filling conditions. The thermosyphon consists of three sections such as evaporator, adiabatic and condenser, which are 300, 300, and 325 mm long, respectively, and the inner diameter is 25 mm. Deionized water is used as a working fluid. Local wall temperatures at both top and bottom surfaces of each section are measured at six different points of the thermosyphon using T-type thermocouples. The internal temperatures at the same points are also measured by inserting the thermocouples into the thermosyphon. Three filling conditions are considered as following degassed, degassed & vacuum and non-degassed. The thermal performance of the thermosyphon is measured at the heat flux ranging from 10 to 300 kW/m^2 and inclination angles of 15°, 45°, and 90°. The results show that the evaporation heat transfer coefficients of three cases are similar at the lower heat flux than 100 kW/m^2. However, at the higher heat flux over 100 kW/m^2, the heat transfer coefficients of the top surface at inclination angle of 15° decrease due to the local dry out. In contrast, the heat transfer coefficients of the condenser section have totally different tendency between degassed and non-degassed cases at the low heat flux up to 150 kW/m^2. It is concluded that the non-condensable gas in the working fluid affects the condensation heat transfer.