This article presents an ultra-wideband(UWB) bandpass filter which used shunt stub with lumped capacitor. Results from transmission line theory and experiments confirmed that a shunt stub with a capacitor can realize a large separation between the first and second resonant frequencies, which then results in a wide passband and compactness. The developed filter has excellent performance: insertion loss of 0.26dB at the central frequency; group delay about 0.13ns; and almost meet the requirement of FCC's UWB spectrum mask except for the band below 2GHz; and compact size of 2cm × 2.5cm.