A microstrip balun-bandpass filter (BPF) using capacitively loaded square-loop dual-mode resonator is proposed; it realizes not only simultaneous size reduction and spurious response suppression in filtering but also overall good differential performance of the balun. The phase and amplitude imbalance improvements due to the loaded resonator are also studied and verified by a prototype balun-BPF operated at 2.55 GHz with 3.5% fractional bandwidth. The experimental results reveal that size reduction of 47% and spurious suppressions of 30 dB beyond 1.5 f0 are achieved as compared to that of the conventional structure. The loading effect of the prototype balun-BPF reports 180.8° and 0.97 dB as phase and amplitude imbalances at the operating frequency respectively.