In this paper, thermal characteristics of cylindrical grooved wick heat pipes with water-based MWCNT nanofluids as working medium are experimentally investigated. Volume fractions of nanoparticles are varied with 0.1% to 0.5%. Transient hot wire method developed in house is used to measure the thermal conductivity of nanofluids. It is enhanced by up to 29% compared to that of DI water. The thermal resistances and temperature distributions at the surface of the heat pipes are measured at the same evaporation temperature. The experimental results show that the thermal resistance of the heat pipes with water-based MWCNT nanofluids as working fluid is reduced up to 35.2% compared with that of heat pipe using DI water. The reduction rate of thermal resistance is greater than the enhancement rate of thermal conductivity. Finally, based on the experimental results, we present the reduction of the thermal resistances of the heat pipes compared with conventional heat pipes cannot be explained by only the thermal conductivity of water-based MWCNT nanofluids.