A theoretical study was made of the flow and heat transfer characteristics in a double tube thermosyphon applied to extract geothermal energy. Water was considered as a working liquid. The total length of the thermosyphon was set to be 1500 m with 10 cm in the inside diameter of the outer tube, 100 m of which was assumed to be the heated length and the rest the unheated length. In our theoretical calculation, the heat input (the heat extraction rate) to the heated section was given, and the circulation mass flow rate was predicted. In order to do that, the distributions of the temperature, enthalpy, pressure, quality, and void fraction were calculated along the flow channels. The effects of the inner tube diameter and the extracted steam temperature on the flow and heat transfer characteristics were examined. Consequently, the critical heat extraction rates were predicted. Also, the heat transfer coefficients and the heated wall temperatures were calculated from the heat, input and the predicted flow rate.