In this paper, a novel dual-mode bandpass filter using square-loop resonator is proposed. The capacitive stepped impedance inner arm is studied so as to achieve simultaneous size reduction and spurious response suppression for the dual-mode bandpass filter. By introducing this new inner arm at the four inner corners along the square-loop resonator, it is found that an effective spurious response suppression measure at 2nd harmonic and 3rd harmonic can be taken. In order to demonstrate the performance of the proposed filter structure, a prototype filter is designed @ 900 MHz with 1% fractional bandwidth. Significant suppressions up to 37 dB and 31 dB are measured @ 1.8 GHz and @ 2.7 GHz respectively. Moreover, a circuitry size reduction of 40% is achieved when compared with that of the conventional structure.