A one-dimensional steady-state mathematical model describing natural circulation two-phase flow in a thermosyphon with tube separator is presented. The theoretical results show that a single liquid region in the lower part of the evaporator is available if the heat rate and operating temperature are low. At high performance operations the total mass flux changes weakly with variations in the operating temperature and heat rate. Void fraction distributions along the thermosyphon length are obtained under various operating conditions. Liquid fill ratios for steady flow are suggested.