A new approach to design a microstrip dual-mode balun bandpass filter (BPF) is proposed in this letter. Its working principle is extensively explained by analysis of standing wave pattern on a half-wavelength λ/2) open-circuited transmission line. By selecting proper coupling topologies between the λ/2 open-circuited transmission line and dual-mode open-stub loaded resonators, a novel planar type dual-mode balun BPF is presented in this letter. To validate the feasibility of the new approach, a dual-mode balun BPF operating at 2.0 GHz with 0.35 dB magnitude imbalance and 5 phase imbalance is designed and fabricated. Both simulated and experimental results are provided with good agreement.