A mobile device, comprising: an antenna structure, comprising: a main radiation element, having a feeding point, wherein a first end of the main radiation element is open, and the feeding point is positioned at a second end of the main radiation element; a first parasitic element, having a first grounding point, wherein the first parasitic element is adjacent to the main radiation element, and wherein the first grounding point is adjacent to the feeding point; and a second parasitic element, having a second grounding point, wherein the second parasitic element is adjacent to the first end of the main radiation element, and wherein each of the main radiation element and the first parasitic element has a straight-line shape, the main radiation element and the first parasitic element are parallel to each other characterized in that the antenna structure operates in a low-frequency band and a high-frequency band, the low-frequency band is from 2400MHz to 2500MHz, and the high-frequency band is from 5150MHz to 5850MHz, wherein a length of the first parasitic element is about 0.25 wavelength of the low-frequency band, and a length of the second parasitic element is about 0.25 wavelength of the high-frequency band, and a length of the main radiation element is about 0.25 wavelength of the low-frequency band, and the mobile device further comprising: a dielectric substrate, wherein the main radiation element and the second parasitic element are disposed on the dielectric substrate; and a metal back cover, comprising a bottom plane and a side wall, wherein the side wall and the bottom plane are perpendicular to each other, wherein the side wall has an opening, wherein the dielectric substrate and the first parasitic element are adjacent to the side wall, wherein the antenna structure has a vertical projection on the side wall, and wherein the vertical projection is at least partially inside the opening. an antenna structure, comprising: a main radiation element, having a feeding point, wherein a first end of the main radiation element is open, and the feeding point is positioned at a second end of the main radiation element; a first parasitic element, having a first grounding point, wherein the first parasitic element is adjacent to the main radiation element, and wherein the first grounding point is adjacent to the feeding point; and a second parasitic element, having a second grounding point, wherein the second parasitic element is adjacent to the first end of the main radiation element, and wherein each of the main radiation element and the first parasitic element has a straight-line shape, the main radiation element and the first parasitic element are parallel to each other characterized in that the antenna structure operates in a low-frequency band and a high-frequency band, the low-frequency band is from 2400MHz to 2500MHz, and the high-frequency band is from 5150MHz to 5850MHz, wherein a length of the first parasitic element is about 0.25 wavelength of the low-frequency band, and a length of the second parasitic element is about 0.25 wavelength of the high-frequency band, and a length of the main radiation element is about 0.25 wavelength of the low-frequency band, and a main radiation element, having a feeding point, wherein a first end of the main radiation element is open, and the feeding point is positioned at a second end of the main radiation element; a first parasitic element, having a first grounding point, wherein the first parasitic element is adjacent to the main radiation element, and wherein the first grounding point is adjacent to the feeding point; and a second parasitic element, having a second grounding point, wherein the second parasitic element is adjacent to the first end of the main radiation element, and wherein each of the main radiation element and the first parasitic element has a straight-line shape, the main radiation element and the first parasitic element are parallel to each other characterized in that the antenna structure operates in a low-frequency band and a high-frequency band, the low-frequency band is from 2400MHz to 2500MHz, and the high-frequency band is from 5150MHz to 5850MHz, wherein a length of the first parasitic element is about 0.25 wavelength of the low-frequency band, and a length of the second parasitic element is about 0.25 wavelength of the high-frequency band, and a length of the main radiation element is about 0.25 wavelength of the low-frequency band, and the mobile device further comprising: a dielectric substrate, wherein the main radiation element and the second parasitic element are disposed on the dielectric substrate; and a metal back cover, comprising a bottom plane and a side wall, wherein the side wall and the bottom plane are perpendicular to each other, wherein the side wall has an opening, wherein the dielectric substrate and the first parasitic element are adjacent to the side wall, wherein the antenna structure has a vertical projection on the side wall, and wherein the vertical projection is at least partially inside the opening. a dielectric substrate, wherein the main radiation element and the second parasitic element are disposed on the dielectric substrate; and a metal back cover, comprising a bottom plane and a side wall, wherein the side wall and the bottom plane are perpendicular to each other, wherein the side wall has an opening, wherein the dielectric substrate and the first parasitic element are adjacent to the side wall, wherein the antenna structure has a vertical projection on the side wall, and wherein the vertical projection is at least partially inside the opening.