Abstract: A heat pipe (10) includes a longitudinal casing (12) having an evaporator section (121) and a condenser section (122), a major wick structure (14) disposed in an inner wall of the casing, at least one assistant wick structure (16) contacting with an inner wall of the major wick structure and extending between the evaporator section and the condenser section, and working medium filling the casing and saturating the major and assistant wick structures. A diameter of a cross section of the assistant wick structure is smaller than a diameter of a cross section of the major wick structure. The assistant wick structure cooperates with the major wick structure to form a composite wick structure, which increases the capillary force inside the heat pipe and further increases the heat transfer capability of the heat pipe.