A flow visualization study of the inside flow phenomena of an inclined, two-phase, closed thermosyphon at several angles has been conducted This article describes the detail of the flow patterns varying with the inclination angle. A thermosyphon with a total length about 930 mm was used, and its evaporator and adiabatic sections were provided with 13-mm-I.D. glass tubing for flow visualization. Freon 113 was charged as the working fluid. Observation of flow phenomena was done at selected inclination angles, such as 90, 80, 70, 60, 55, 45, 35, 20, and 5 degrees. Flow phenomena were recorded with a standard video camera and a still camara, and the corresponding heat transfer rate was also measured It was found that the typical flow patterns at several angles are classified by inclination angle, such as an annular flow at vertical or very near the vertical position, and a stratified flow at other angles. Moreover, in the case of a stratified flow, the occurrence of dryout on the evaporator wall is caused by the depletion of working fluid in the evaporator section as a result of the development of the interfacial liquid wave.