A heat pipe (10) includes a metal casing (12) having an evaporator section (121) and a condenser section (122). A major wick structure (14) is disposed on an inner wall of the casing. At least one assistant wick structure (16) is disposed in and contacts with the major wick structure, and working medium fills in the casing and saturates the major wick structure and the at least one assistant wick structure. An average pore size of the major wick structure corresponding to the evaporator section is smaller than that of the major wick structure corresponding to the condenser section. A diameter of a cross section of the at least one assistant wick structure is smaller than a diameter of a cross section of the major wick structure, and thus the at least one assistant wick structure has a linear contact with the major wick structure.