Experiments are conducted to determine the effect of dimensionless parameters on heat-transfer characteristics of a vertical flat thermosyphon (VET). The VET used in this study employed a copper tube with an inner diameter of 8.6 mm. The cross-sectional areas of the tube were reduced by pressing them into a mold at pressed thicknesses of 6.6, 4.6 and 2.6 mm. The evaporator, adiabatic and condenser were similar in lengths of 0.13, 0.23 and 0.33 m, respectively. The working fluids used were R123, ethanol and water with filling ratios of 20%, 40%, 60% and 80% of the total volume. The evaporator section was heated with hot water while the condenser section was cooled with water. When the system reached an appropriate flow rate volume, the temperatures of the cooling water were recorded in order to calculate the heat-transfer rate of the VFT. After this, the dimensionless parameter relationships were analyzed from the experimental results. After the study processes were completed, a correlation of the dimensionless parameters for predicting VET heat-transfer rate was established. (C) 2010 Elsevier Ltd. All rights reserved.