In this paper, a novel millimeter-wave ultra-wideband (UWB) bandpass filter having a midband frequency of 26.5 GHz and a 3 dB bandwidth of 3.4 GHz is developed by using microstrip dual-mode rectangular ring resonators. Resonant properties of the microstrip dual-mode rectangular ring resonator are analyzed by using an electromagnetic simulator, and stepped- impedance parallel-coupled feed lines are invented in order to realize strong coupling between the input/output lines and the dual-mode ring resonators. The dual-mode ring resonators produce multiple transmission zeros in the stopband of the filter, as a consequence, the filter exhibits sharp attenuations near its passband, and very wide stopbands. The filter is compact in size and low-loss in passband. The measured frequency property of a fabricated filter shows favorable agreement with the predicted response.