Abstract: A heat pipe comprising a hermetic shell tube, a heat conveying fluid within the shell tube and a plurality of capillary circumferential or axial grooves provided entirely of an inner surface of the shell. A capillary axial channel structure which includes axially extending plates defining a U-shaped or V-shaped channels therebetween circumferential grooves and having an opening defining therein a meniscus of the heat conveying fluid in the liquid phase at least in the evaporator section and the condenser section. The capillary axial channel structure may comprise an inner tube for defining a tubular, axially extending capillary space therebetween. The inner tube has open ends disposed within the evaporation and condensation sections for allowing the heat conveying fluid to flow therethrough.