This project studied the effect of aspect ratios which influence the flow patterns inside an inclined closed two-phase thermosyphon at the critical state. The studied thermosyphon consisted of a condenser section made of a copper tube, an evaporator section and adiabatic section made of a removable transparent glass tube, with inner diameters of 10, 12 and 28.5 mm. Silicone oil continuously transferred heat from a hot bath to the evaporator jacket which was made of a transparent glass tube that enclosed the evaporator section, A mixture of equal amount of water - ethylene glycol continuously received heat from a cold bath and the plastic tube that enclosed the condenser section. R - 123 was used as the working fluid, and the filling ratio was chosen as 80 % of the evaporator volume. The experiments were conducted at inclination angles of 90, 5 and the angle of maximum heat flux (60 degrees) from the horizontal plane with aspect ratios of 5, 10, 20 and 30. The volume flow rate of cool water - ethylene glycol mixture was maintained at 2 L / min and at -10 oC for the inlet temperature. The hot silicone oil temperatures were periodically varied by 10 oC. The temperatures of the cool water - ethylene glycol mixture at the inlet and the outlet part of the condenser section, were recorded. The flow patterns at the steady state at all of the hot bath temperatures were recorded by a still camera. Also, the flow patterns were continuously recorded by a video camera to observe the side flow patterns . The results showed that : 1. Inner flow patterns and characteristic occurrence of dryout at the critical stateWith the aspect ratios of less than 10, the flow patterns at the critical state were bubble flow for a vertical tube but slug flow for an inclined angle tube. The occurrence of dryout was caused by film boiling .With the aspect ratios equal to or greater than 10, the flow patterns were annular flow at vertical but stratified flow for inclined angles. The occurrence of dryout was caused by flooding phenomenon at vertical angle and similarly the critical heat flux value had no effects on flow patterns. 2. Effects of aspect ratios on critical heat flux valueWith aspect ratios of less than 10, at vertical, the flow patterns at the critical state were bubble flow but slug flow for inclined angles which similarly caused dryout due to film boiling phenomena. However, with the aspect ratios equal to or greater than 10, the flow patterns at vertical were annular flow but stratified flow for inclined angles which similarly caused dryout due to flooding phenomena. Moreover, when the method of statistic collection was considered it was found that the aspect ratios has no effect on Qc/Qc90.Experiment result showed good correlation between the aspect ratios and critical heat flux at vertical in corresponding with the equation of Tein and Chung, Nejat, Katto and Imura et al. And the aspect ratios were inversely proportional to the critical heat flux.From the correlation between Qc/Qc90 and Ku of the experiment data there was also noted a correlation in the form of 3. Flow pattern map The flow pattern map can predict the flow patterns in the inner thermosyphon and Ku* can be calculated from the equation.