This paper presents a two-dimensional CFD model designed to simulate two-phase flow, evaporation, condensation, and heat transfer within a thermosyphon. The simulation utilized the mixture model in ANSYS FLUENT 19.2. Water and ethanol were chosen as working fluids to evaluate thermosyphon performance. The evaporator section was subjected to a constant heat flux with inputs of 200, 400, and 600 W, while the condenser section had three different heat transfer coefficients: 200, 500, and 1000 W/m2 K. Due to ethanol’s lower latent heat compared to water, simulations were performed with heat inputs of 100 and 200 W. Additionally, three fill ratios of 25, 50, and 75% were examined. The study numerically assessed the overall effective thermal resistance and temperature distribution along the length of the thermosyphon. It has been observed that, fill ratio, heat flux, and condenser side heat transfer coefficient significantly affect the overall thermal resistance of the thermosyphon.