A balun filter utilized for a radio-frequency (RF) system includes a first terminal coupled to an antenna of the RF system for delivering an RF signal; a differential port has a second terminal and a third terminal for delivering a differential signal; a band pass filter coupled between the first terminal and the differential port has a plurality of resonators, each including a surrounding line substantially surrounding an area and forming a loophole on a side of the each resonator; and at least a line segment connected to the surrounding line and disposed separately within the area surrounded by the surrounding line.