A heat transfer experiment was performed to investigate the heat transfer performance of a small thermosyphon using carbon nanotube（CNT）suspensions. The study was focused on the effects of the mass concentration of the CNT suspension and the operating pressure of the thermosyphon on the heat transfer and critical heat flux（CHF）in the evaporator. The CNT concentration ranged from 0. 1% to 3%(mass) and the operating pressures of the thermosyphon varied from 7. 45 kPa to 19. 97 kPa. The experimental results showed that CNT suspensions could significantly enhance heat transfer coefficient and CHF in the evaporator under sub-atmospheric pressures. Under the pressure of 7. 45 kPa，the heat transfer coefficient and CHF of the evaporator increased by 40 % and 120 % when water was replaced by 2. 0% CNT suspension. Wall heat flux had remarkable influence on the heat transfer of CNT suspensions. The enhanced heat transfer effect was weak at a low heat flux and it gradually increased with the increase of wall heat flux.