A heat pipe includes an outer pipe (10), an inner pipe (20), and a hermetic cap (30). The outer pipe has an evaporating end (12) and a condensing end (14). The evaporating end is integrally sealed and receives working fluid. The inner pipe includes an open top and an open bottom. A very narrow gap (40) is defined between the inner pipe and the outer pipe. A plurality of granules is put into the gap to form a porous wicking structure. When the evaporating end is heated by an external heat source, the working fluid is vaporized and flows up along the inner pipe to the condensing end. The working fluid condenses at the condensing end, and flows back down to the evaporating end through the gap. Because the gap is very narrow, surface tension of the working fluid and capillary action of the outer and inner pipes is enhanced.