Heat pipes are passive devices that use the phase change of a working fluid to transfer heat between two surfaces. In the past decades, Heat Pipes have gained enormous attention in the field of thermal management of electronic devices. The present study deals with the numerical investigations of a wickless Heat Pipe (Thermosyphon) that utilizes Copper Oxide (CuO) Nano-fluid as the working fluid. The Heat Pipe employing Deionized Water is considered as the base fluid for comparison. The Nano-particles are added at different concentrations (1%, 5%, and 10%) to the base fluid, and the heat input to the Heat pipe is varied between 10-50 W in steps of 10 W. The results indicated that the addition of Nanoparticles to a Heat Pipe improved significantly its heat transfer characteristics. The evaporator heat transfer coefficient and effective thermal conductivity of the Heat Pipe are enhanced by 26% and 5%, respectively. Also, the temperature difference between the evaporator and condenser is decreased by 4.5%.