Abstract: A design of miniaturized bandpass filter microstrip balun is presented in this paper. On the basis of Marchand balun, using one half wavelength and two quarter wavelength transmission lines as resonators, the balanced signals with the same amplitude and opposite phase can be obtained at the two output ports of bandpass filter microstrip balun. Two transmission zeros on both sides of the passband can be observed by using the parallel line coupling of the resonator. The proposed bandpass filter microstrip balun with the center operating frequency of 2. 65 GHz is fabricated and measured. The amplitude and phase difference of the dual port output signal are ±0.4 dB and 180°±5° respectively. The simulation results are in good agreement with the measurement results, which also verifies the proposed scheme.