This paper reports a novel multimode resonator (MMR) ultra-wide band-pass (UWB) filter design using a grounded open ring resonator. The utilization of 30 μm gaps between input ports and the central resonator achieve high coupling level. Five resonant modes are used to perform a 128% fractional bandwidth at 4.3 GHz. The fabricated prototype exhibits a band-pass with insertion losses around 0.5 dB. A theoretical model based on transmission lines matrices is proposed and used to explore the limits of the design through different parametric analysis. Excellent agreement between theoretical predictions, electromagnetic simulations and measurement are found.