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In this paper, a novel nanometeric plasmonic narrow-band filter consisting of a metal-insulator-metal waveguide and a cavity embedded in the metal is proposed and numerically demonstrated by using the finite difference time domain method. The physical mechanism of the filter is elucidated. The results reveal that a narrow-band spectrum notch-filtering function can be realized with the proposed structure. The selection wavelength of the notch is proportional to the length of the cavity and inversely quasi-proportional to its width. The simple plasmonic filter structure has an ultracompact size with a length of a few hundred nanometers. Our results may open a way to construct nanoscale high-density photonic integration circuits based on plasmonic effects.