Abstract: We propose and numerically investigate an ultra-compact surface plasmon polaritons (SPPs) band-pass filter based on a rectangular cavity with two half circle ends. The transmission properties of the filter are simulated by the Finite-Difference Time-Domain (FDTD) method. Attributed to the half circle ends, the transmittances can be optimized with the same band width. One can realize the required transmission spectra by adjusting the radius of the half circles and the length of the cavity. Moreover, with a section of waveguide added on the output waveguide at the bottom of the cavity, a 12 wavelength demultiplexer with the ability of inhibiting resonant mode is proposed. And by embedding the liquid crystal in the resonator, the ?lter can act as an electro-optical switch. These results show potential applications on nano-optical devices.