A multiband built-in antenna for a portable terminal is provided, which includes a feeding part electrically connected to a Radio Frequency (RF) connector of a terminal main board; an antenna radiator comprising at least two ground parts electrically connected to a ground means of the terminal and formed at different locations; and a switching means commonly linked to a ground line of each ground part of the antenna radiator, and selectively connecting one or more of the at least two ground parts electrically to the ground means according to a switching operation