A novel microwave planar triple-mode resonator filter structure to achieve triple-mode rather than dual-mode response has been developed for miniaturization of high selectivity narrow-band microwave bandpass filter. The filter is miniaturized by incorporating a novel stepped impedance triple-mode into resonator. The filter has a passband bandwidth of 6 at 1 GHz. The measured result shows less than 1.1 dB passband insertion loss and more than 30 dB return loss are obtained. © 2011 IEEE.