We effects of Bond number (Bo) on internal flow patterns of an inclined, closed, two-phase thermosyphon at critical state are considered in this article. The thermosyphons used employed a glass tube for the evaporator and the adiabatic section, and a copper tube for the condenser section. The internal diameters of the thermosyphon were 10, 12, and 28.5 mm. The aspect ratios were 5 and 30, with inclination angles of 90 degrees 60 degrees, and 5 degrees (against the horizontal axis). R123 was used as working fluid, with a filling ratio of 80% and 150% of the evaporator section. A solution of water and ethylene glycol was used to carry heat from the condenser, while water was employed to supply heat to the thermosyphon. A video camera was used to record all the flow patterns, while a still camera,uas used to record the internal flow patterns at specific times. Heat transfer rate was measured by means of a calorimeter in the condenser section. It was found from the experiments that, as under normal operating conditions, an Le/d of 10 is an important value at which changes of internal flow patterns can be observed. At an Le/d < 10 and with low Bo, the internal flow pattern at critical state changed from bubble flow at vertical to slug flow at an inclined position. Although the flow patterns were the same as,with high Bo, the bubbles generated were smaller and appeared only on the upper side. In cases of Le/d > 10, the pattern changed from annular flow at vertical to stratified flow at inclined positions for all Bo. Generated bubbles,were comparatively small at low Bo. A flow pattern map at each angle was also created by using modified Kutateladze numbers and Reynolds numbers of the vapor.