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In this letter, a miniaturized tri-band bandpass filter (BPF) using a multimode stepped-impedance resonator (SIR) with a 0 ° tapped-feed structure is presented. The odd- and even-mode fundamental resonant frequencies and the first high-order resonant frequencies of the SIR are utilized to generate dual-mode dual-band response and the tapped side-coupled feed-lines construct an additional passband avoiding occupying extra size. Owing to the 0 ° tapped-feed structure and the intrinsic characteristics of the SIR, multiple transmission zeros are created to improve the selectivity of the filter. The bands of the designed tri-band BPF are located at 1.57 GHz (GPS application), 3.5 GHz (WiMAX application) and 5.8 GHz (WLAN application). Good agreement between the simulated and the measured results is observed.