A communication device, comprising: a ground element; a substrate, adjacent to the ground element; and an antenna, wherein the antenna provides a first band and a second band, and the antenna is disposed on the substrate, the antenna comprises: a radiator; a feed conductor, wherein an end of the feed conductor is connected to a signal source, and another end of the feed conductor is electrically connected to the radiator; a capacitor unit, connecting the feed conductor to the radiator; and a short-circuiting unit, comprising a first short-circuiting element and a second short-circuiting element, wherein both the first and the second short-circuiting elements (16, 17) connect the radiator to the ground element, wherein the short-circuiting unit is entirely located between the radiator and the ground element, the first short-circuiting element has a first path length, the second short-circuiting element has a second path length, the first and second path lengths are longer than 0.05 times that of a wavelength of a lowest frequency of the first band, and the first short-circuiting element is substantially L-shaped, the second short-circuiting element is substantially L-shaped, and the first short-circuiting element and the second short-circuiting element define a rectangular slot. a ground element; a substrate, adjacent to the ground element; and an antenna, wherein the antenna provides a first band and a second band, and the antenna is disposed on the substrate, the antenna comprises: a radiator; a feed conductor, wherein an end of the feed conductor is connected to a signal source, and another end of the feed conductor is electrically connected to the radiator; a capacitor unit, connecting the feed conductor to the radiator; and a short-circuiting unit, comprising a first short-circuiting element and a second short-circuiting element, wherein both the first and the second short-circuiting elements (16, 17) connect the radiator to the ground element, wherein the short-circuiting unit is entirely located between the radiator and the ground element, the first short-circuiting element has a first path length, the second short-circuiting element has a second path length, the first and second path lengths are longer than 0.05 times that of a wavelength of a lowest frequency of the first band, and the first short-circuiting element is substantially L-shaped, the second short-circuiting element is substantially L-shaped, and the first short-circuiting element and the second short-circuiting element define a rectangular slot. a radiator; a feed conductor, wherein an end of the feed conductor is connected to a signal source, and another end of the feed conductor is electrically connected to the radiator; a capacitor unit, connecting the feed conductor to the radiator; and a short-circuiting unit, comprising a first short-circuiting element and a second short-circuiting element, wherein both the first and the second short-circuiting elements (16, 17) connect the radiator to the ground element, wherein the short-circuiting unit is entirely located between the radiator and the ground element, the first short-circuiting element has a first path length, the second short-circuiting element has a second path length, the first and second path lengths are longer than 0.05 times that of a wavelength of a lowest frequency of the first band, and the first short-circuiting element is substantially L-shaped, the second short-circuiting element is substantially L-shaped, and the first short-circuiting element and the second short-circuiting element define a rectangular slot.