The enhancement in heat transfer characteristics of a copper sintered wick heat pipe with surfactant free CuO nanoparticles dispersed in DI water is experimentally studied. The effect of heat input, tilt angle and weight fractions of nanoparticles on the heat pipe thermal resistance, heat transfer coefficient in evaporator and condenser sections, thermal conductivity and thermal efficiency are investigated. The experimental results are evaluated for the vapor temperatures directly measured at the center core of heat pipe. Interestingly, a temperature difference of 5.1 degrees C is observed between the heat pipe surface and the vapor core in the evaporator section. Results showed a reduction in the thermal resistance of 66.1% and enhancement in the heat transfer coefficient and thermal conductivity of 29.4% and 63.5% is respectively, observed for 1.0 wt.% of CuO/DI water nanofluid at 45 degrees tilt angle compared with heat pipe kept at horizontal position. Similarly, the thermal efficiency is also improved by 24.9% for the same tilt angle and weight fraction of CuO nanofluid. The use of nanoparticles and tilt angle enhances the operating range and thermal performance of heat pipe when compared with that of the heat pipe with DI water. (c) 2014 Elsevier Ltd. All rights reserved.