Abstract: An experiment was performed to investigate the thermal performance of inclined miniature grooved heat pipe using water-based CuO nanofluids.The study focused on the effects of the inclination angle and the operating pressure on the heat transfers of both the evaporator and condenser as well as the maximum heat flux of the heat pipe.The nanofluid consists of pure water and CuO nanoparticles with a mean diameter of 50nm.The mass concentration of the nanofluid was fixed at an optimal value of 1.0%.The experimental results show that the inclination angle has a very significant effect on both the total heat resistance and maximal power of the heat pipe using water,and only a slight effect on those of the heat pipe using nanofluids.There exists an optimal inclination angle which corresponds to the best thermal performance,and this angle is 75° for both heat pipes using water and nanofluids.For the inclined heat pipe using nanofluids,not only the evaporation and condensation heat transfer coefficients are greatly increased,but also the maximal power can be almost doubled as compared with those of the heat pipe using water.The present investigation discovers that the thermal performance of an inclined miniature grooved heat pipe can be evidently strengthened by using CuO nanofluids.