This paper presents an ultra-wideband impedance transforming balun bandpass filter in organic packaging form by using multilayer liquid crystal polymer (LCP) technology. The broadside coupled stripline structure has been adopted in our design to realize ultra-wideband performance. Two-port equivalent circuit with asymmetrical port impedances is given and the synthesized results show that the proposed balun filter can achieve a three-pole Chebyshev equal-ripple response. For verification, a prototype centered at 2.0 GHz with 50-200 Ω impedance transformation and 3-dB bandwidth of 125% is designed. Theoretical and simulated results are given and good agreement is obtained.