A heat pipe includes a metal tube, a layer of nano-particles and a working fluid. The metal tube has an inner surface defining at least two pores. Each of the pores has a diameter in the range from about 2 to about 50 nanometers. The layer of nano-particles is formed at least one of on the surfaces of the meso-pores and on the inner surface of the metal tube. The working fluid is in the metal tube.