Abstract: An experiment was carried out to study the heat transfer performance of an axially microgrooved heat pipe using water based multiwalled carbon nanotube (MWNT) suspensions as the working fluid. The effects of the MWNT mass concentration and the operating pressure on the evaporation and condensation heat transfer coefficients, the maximum heat flux and the total heat resistance of the heat pipe were investigated and discussed. The experimental results show that MWNT suspensions can apparently improve the thermal performance of the heat pipe and there is an optimal MWNT mass concentration of about 2.0 % to achieve the maximum heat transfer enhancement. The operating pressure has a significant influence on the enhancement of heat transfer coefficients, and slight influences on the enhancement of the maximum heat flux. Under the pressure of 7.45 kPa, the evaporation heat transfer coefficient and the maximum heat flux for the MWNT suspension can be enhanced up most by about 80% and 25% respectively compared to those for water.