An ultra-compact surface plasmon polaritons( SPPs) band-pass filter based on a rectangular cavity with two half circle ends is proposed and numerically investigated. 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 s...