A reconfigurable tri/quad-band resonator and the application to tri/quad-band band-pass filter have been demonstrated. By changing the switching state of the PIN diodes, the filter can work in tri-band mode or quad-band mode. Correspondingly, the resonant frequencies of the band-pass filter are 1.21 GHz, 2.0 GHz, 3.23 GHz, 1.27 GHz, 2.0 GHz, 2.61 GHz, and 3.41 GHz, respectively. Especially, the filter has a combined structure of lumped capacitors and some transmission lines and achieves a small mechanical size of 0.14 λg
× 0.09 λg
, where λg
denotes the wavelength on the substrate of the filter at the first central frequency. The measurement results of the fabricated filter agree with the electromagnetic simulation data.