Abstract: A compact ultra-wideband (UWB) bandpass filter (BPF) with improved upper-stopband performance using novel radial-stub-loaded resonator (RSLR) is proposed. The multiple-mode resonator (MMR) is formed by loading three pairs of radial-stubs in shunt to a high impedance microstrip line. By properly adjusting the dimensions of these radial-stubs, the resonant modes of RSLR can be roughly allocated in the UWB passband (3.1~10.6 GHz). To enhance the coupling degree, two interdigital coupled feed lines are utilized in this filter. The design steps of this filter have also been presented. Finally, a prototype of the proposed filter has been designed and fabricated. The measured results are in good agreement with full-wave simulated results.