A visual study of internal flow patterns inside an inclined, closed, two-phase thermosyphon at normal operating conditions for several Bond numbers has been conducted This article describes the effects of varying inclination angles and Bond numbers (Be) on flow patterns. A copper thermosyphon was used. R123 was selected as the working fluid, Kith a vapor temperature of 30 degrees C and a filling ratio of 50%. Flow phenomena were observed at selected inclination angles of 90 degrees and 30 degrees from the horizontal axis, with aspect ratios of 30 and 5. The selected external diameters were 12.0 and 28.5 mm. Flow phenomena were also recorded with a standard video camera and a still camera, and the corresponding heat transfer rates were also monitored. It was observed that the basic internal flow patterns could be classified according to the aspect ratios and the Bo as follows: At an aspect ratio of 10 and higher, the annular flow at vertical changes to slug flow in inclined positions for all values of Bo. A thin liquid film always exists in the upper part, promoting heat transfer. In this case, the ratio of heat transfer rates at peak position and those of the vertical (Q/Q90) are almost the same for all Bo. At an aspect ratio of less than IO and a low Bo, bubbly flow at vertical changes to stratified flow at an inclined position, with bubbly flow in the lower part Vapor slug is sometimes seen. At an aspect ratio of less than 10 and a high Bo, bubbly flow with a coagulation of bubbles at the vertical position changes to stratified flow with bubbly flow in the upper part, without any vapor slugs. Q/Q90 is comparatively higher than at lower Bo. This result may be attributed to two main factors: (I) there was an area undisturbed by bubbles in the lower part; and (2) no upward vapor slug movement was observed.