A monopole antenna is disposed on a substrate including a first surface and a second surface. The monopole antenna includes a feeding point, a radiation unit, and a reflecting element. The radiation unit is disposed on the first surface of the substrate, and includes a feeding section, a first radiation section, a second radiation section, and a third radiation section. The feeding section, the first radiation section, and the second radiation section are connected sequentially. The feeding point is electrically connected to the feeding section. The second radiation section and the feeding section are respectively placed at two sides of a longitudinal axis of the first radiation section. The third radiation section is electrically connected to the first radiation section. The reflecting element is disposed on the second surface of the substrate, and corresponds to a position of the second radiation section.