This paper presents a miniaturized balanced-to-unbalanced transformer (balun) with bandpass responses. Two miniaturized K-inverters with filtering components are applied to substitute for the λ/4 transmission (λ is the wavelength in vacuum) line of the conventional balun structure, which can reduce the size and have good out-of-band suppression performance. The relevant theoretical analysis and specific steps of method realization are listed. To illustrate, a filtering balun operating at 400 MHz, which is widely used in mobile communication, is implemented. Bandwidth and insertion loss is 15.63 % and 1.2 dB, respectively. In the passband, the two outputs have identical amplitudes, and exhibit a phase difference of 180 degrees. The circuit size is 0.051 λ × 0.033 λ, which is compact.