A compact size antenna operating in LTE frequency bands, comprising: a radiation element (12, 42), at least comprising a first radiation branch, wherein the first radiation branch extends toward a first direction, and a connection end of the radiation element (12, 42) comprises a signal feeding point (120, 420); a ground plane; a connecting piece, coupled to the ground plane; and a ground extension element, comprising: a metal arm, coupled to the ground plane through the connecting piece; a first ground branch, coupled to the metal arm, and extending toward the first direction; a second ground branch, coupled to the metal arm, and extending toward a second direction opposite to the first direction; and a third ground branch, coupled to the metal arm and the second ground branch, and extending toward the first direction, wherein the radiation element (12, 42) is disposed on a first plane; the ground extension element is disposed on a second plane perpendicular to the first plane; and the connecting piece is disposed on a third plane perpendicular to the first plane and the second plane, the compact size antenna being characterized in that the radiation element (12, 42) further comprises: a second radiation branch, coupled to the connection end of the first radiation branch, and extending toward the first direction. a radiation element (12, 42), at least comprising a first radiation branch, wherein the first radiation branch extends toward a first direction, and a connection end of the radiation element (12, 42) comprises a signal feeding point (120, 420); a ground plane; a connecting piece, coupled to the ground plane; and a ground extension element, comprising: a metal arm, coupled to the ground plane through the connecting piece; a first ground branch, coupled to the metal arm, and extending toward the first direction; a second ground branch, coupled to the metal arm, and extending toward a second direction opposite to the first direction; and a third ground branch, coupled to the metal arm and the second ground branch, and extending toward the first direction, wherein the radiation element (12, 42) is disposed on a first plane; the ground extension element is disposed on a second plane perpendicular to the first plane; and the connecting piece is disposed on a third plane perpendicular to the first plane and the second plane, the compact size antenna being characterized in that the radiation element (12, 42) further comprises: a metal arm, coupled to the ground plane through the connecting piece; a first ground branch, coupled to the metal arm, and extending toward the first direction; a second ground branch, coupled to the metal arm, and extending toward a second direction opposite to the first direction; and a third ground branch, coupled to the metal arm and the second ground branch, and extending toward the first direction, wherein the radiation element (12, 42) is disposed on a first plane; the ground extension element is disposed on a second plane perpendicular to the first plane; and the connecting piece is disposed on a third plane perpendicular to the first plane and the second plane, the compact size antenna being characterized in that the radiation element (12, 42) further comprises: a second radiation branch, coupled to the connection end of the first radiation branch, and extending toward the first direction.