A monopole slot antenna applicable to a mobile communication device includes a dielectric substrate, a first ground plane, a second ground plane, a monopole slot, and a microstrip feedline. The first ground plane is disposed on the dielectric substrate. The second ground plane is in the vicinity of the first ground plane and electrically connected to the first ground plane via a metal wire. A section of the metal wire is disposed on one surface of the dielectric substrate. The monopole slot is disposed on the first ground plane and has an open end disposed near the metal wire that connects the first and the second ground planes. The microstrip feedline is disposed on a surface of the dielectric substrate opposite to the first ground plane with one end of the microstrip feedline extended across the monopole slot and the other end connected to a signal source.