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A compact quadruple-mode square ring loaded resonator (SRLR) is proposed in this letter. Distinct mode splitting characteristics of the quadruple-mode SRLR are investigated and explained by using even- and odd-mode analysis. It results to excite four resonant modes which are applied to produce two pairs of transmission poles for dual-band bandpass filter (BPF) design. Based on the unique characteristics of the proposed SRLR, a miniaturized dual-band BPF operating at 2.45 GHz and 5.20 GHz for WLAN applications was designed and fabricated. Multiple transmission zeros are generated to improve the selectivity of the filter. Measurements are in good agreement with the simulations. Also, the circuit size occupies only 0.11λg×0.23λg, where λg is the guided wavelength at the center frequency of the first passband.