This article describes the effect of aspect ratio (Le/d) on internal flow patterns of an inclined, closed, two-phase thermosyphon at critical state. The thermosyphon used copper tubing as a condenser, and the evaporator and the adiabatic section were made of a glass tube with an internal diameter of 10 mm. The aspect ratios were 5, 10, 20, 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. Silicone oil was employed to supply heat to the thermosyphon, while a solution of water and ethylene glycol was used to carry the heat from the condenser. A still camera was used to record the internal flow patterns at specified times and a video camera,uas used to record all the flow patterns. The heat transfer rate was also measured by means of a calorimeter positioned at the condenser section. It was found from the experiments that, as in normal operating renditions, the Le/d of 10 was an important value at which changing of internal flow patterns can be realized. At Le/d < 10, the internal flow pattern at critical state changed from bubble flow at vertical to slug flow at an inclined position, but at Le/d > 10, the pattern changed from annular flow at vertical to stratified flow at an inclined position. However, in qualitative consideration, Le/d did not affect the ratio of the maximum critical heat transfer rate at an inclined position to that of vertical (Q/Q(90)) though the Q(90) and Q/Q(90) are statistically close to the predicted values of many widely accepted correlations.