A three-dimensional antenna structure is applicable to a transmission frequency band, and includes an insulating carrier, an M number of first antennas, an M number of microstrip lines, a second antenna, and a grounding element. The M microstrip lines are electrically coupled to the M first antennas. The second antenna is electrically coupled to the M first antennas through a connection point of each of the M microstrip lines, and has an M−1 number of line segments. Any end of each of the M−1 line segments correspond in position to one of the connection points of the M microstrip lines. A length of each of the M−1 line segments is 1/M times a wavelength corresponding to a center frequency of the transmission frequency band. The grounding element is electrically coupled to the M first antennas through the M microstrip lines.