Abstract: Since the wavelength feature of an optical filter is fixed once it is made, surface plasmon polarition on a flat metal-dielectric interface is used to make a modulated filter. When light beam is focused on the metal-dielectric interface, the surface plasmon polarition (SPP) of metal-dielectric will be changed to modulate the wavelength of the filter by modulating the dielectric index of the dielectric layer. A modulated optical filter is fabricated on an Au film with a set of sub-wavelength nanometer hole arrays. In the experiment, air, alcohol and oil are selected as dielectric layers to test the filter. Unlike the conventional optical filter, this filter can be modulated continuously and conveniently.