In the present study, the enhancement of heat pipe efficiency with refrigerant-nanoparticles mixtures is presented. The heat pipe is fabricated from the straight copper tube with the outer diameter and length of 15, 600 mm, respectively. The refrigerant (R11) is used as a base working fluid while the nanoparticles used in the present study are the titanium nanoparticles with diameter of 21 nm. The mixtures of refrigerant and nanoparticles are prepared using an ultrasonic homogenizer. Effects of the charge amount of working fluid, heat pipe tilt angle on the efficiency of heat pipe are considered. For the used pure refrigerant as working fluid, the heat pipe at the tilt angle of 60 degrees, working fluid charge amount of 50% gives the highest efficiency. At the optimum condition for the pure refrigerant, the heat pipe with 0.1% nanoparticles concentration gives efficiency 1.40 times higher than that with pure refrigerant. (C) 2008 Elsevier Ltd. All rights reserved.