A mathematical model for predicting the heat-transfer performance in a heat sink embedded with a closed two-phase thermosyphon is presented. The model includes boiling heat transfer in the evaporator, vapor flow influence on condensation heat transfer, and the effects of fin surface areas and airflow rates on the convection heat transfer. The results obtained from the model indicate that the heat-transfer resistance occurring in the evaporator is the primary factor affecting the total temperature drop in the heat sink embedded with a thermosyphon. Although the effect of vapor flow on the condensation heat transfer can be neglected, the condensation heat transfer in the condenser significantly depends on the type of working fluid. In addition, a new relationship between heat transfer and flow characteristics in terms of the Colburn factor j H and the Reynolds number was developed to predict the forced airflow effect on the fin-tube condenser's heat-transfer performance. A correlation for boiling heat transfer occurring in the evaporator was also determined experimentally. To verify the prediction, an experimental investigation was conducted to measure the temperature drops from the evaporator to the forced airflow in the heat sink embedded with a thermosyphon charged with acetone. The results of this investigation will assist in the development of high heat-flux cooling devices capable of operating at power levels up to 120 W/cm 2 with a low temperature drop.