Pulsating heat pipes (PHPs) have emerged as possible cooling devices for high heat flux electronics. In tliis study, an optimum PHP design is investigated. The PHPs were constructed of multiport extruded alum inum tubing. Several parameters were varied for this purpose:​ type of working fluid, the fluid fill ratio, the heat pipe orientation, and the PHP dimensions. Addition of diamond nanoparticles to the working fluid was also tested. It was found that the optimum fluid fill ratio depends on the PHP structure, with 50% and 20% being the best values for the 35-port and 26-port PHPs, respectively. Also, the 35-port design was able to dissipate a higher power input of 80W, versus 60W for the 26-port case. In the vertical orientation the PHP delivered better performance compared to the horizontal case. Acetone outperformed ethanol only at low power inputs. The nanoparticle addition slightly improved the PHP performance.