A novel wideband balun filter on multi-mode slotline resonator topology with good balanced performance in magnitude and phase is reported in this paper. The proposed wideband balun filter consists of one uniform half-wavelength slot-line resonator (SLR), one single-ended unbalanced port, and one balanced port with two terminals. By utilizing the distinct field conversion from the slotline to the microstrip feeding line, the proposed balun filter can realize good balanced performance in magnitude and phase over a wideband frequency range. In order to demonstrate the proposed scheme and support the design conception, one wideband balun filter prototype is designed, fabricated, and tested. All of the measured results of frequency response, magnitude and phase imbalance performance agree well with the simulated one, and have effectively demonstrated the good frequency selectivity and balanced performance in both magnitude and phase over a wide frequency band.