An experimental study was carried out to understand the heat transfer performance of a miniature thermosyphon using water-based carbon nanotube (CNT) suspensions as the working fluid. The suspensions consisted of deionized water and multi-wall carbon nanotubes with an average diameter of 15 nm and a length range of 5-15 lm. Experiments were performed under three steady operation pressures of 7.4 kPa, 13.2 kPa and 20 kPa, respectively. Effects of the CNT mass concentration and the operation pressure on the average evaporation and condensation heat transfer coefficients, the critical heat flux and the total heat resistance of the thermosyphon were investigated and discussed. Experimental results show that CNT suspensions can apparently improve the thermal performance of the thermosyphon and there is an optimal CNT mass concentration (about 2.0%) to achieve the maximum heat transfer enhancement. The operation pressure has a significant influence on the enhancement of the evaporation heat transfer coefficient, and slight influences on the enhancement of the critical heat flux. The enhanced heat transfer effect is weak at low heat fluxes while it is increased gradually with increasing the heat flux. The present experiment confirms that the thermal performance of a miniature thermosyphon can be strengthened evidently by using CNT suspensions.