A slim mobile communication device includes an antenna structure. The antenna structure includes a dielectric substrate, a ground element, an antenna element, and a feeding line. The antenna element is a planar structure and is disposed on a no-ground portion of the dielectric substrate. At least two edges of the no-ground portion are surrounded by a ground element of the dielectric substrate, wherein one of the edges used as a feeding edge and the other edges are non-feeding edges. A distance between the non-feeding side edge of the antenna element and the second edge of the no-ground portion is smaller than 3 mm. A length of the non-feeding side edge of the antenna element is at least 5 mm. The non-feeding side edge of the antenna element is short-circuited to the ground element. The feeding line is coupled to the feeding side edge of the antenna element.