To provide a water-based working fluid for a heat pipe capable of markedly improving a heat transport characteristic of the heat pipe, and to provide the heat pipe with a markedly improved heat transport characteristic.

A working fluid for a heat pipe is characterized in that alkyl polyglycoside is added to water therein. Preferably concentration of the alkyl polyglycoside is 50 to 50,000 ppm (volumetric basis). Further, 0.01 to 20 wt.% of alcohol of carbon number 3 or more is added if required. Further, metal or metal-oxide nanoparticles at concentration of 10 to 10,000 ppm (volumetric basis) is added if required.