We use two parallel nano-slits in a silver film to form a surface plasmon Mach-Zehnder interferometer (MZI),based on the interference of two surface plasmon waves propagating along the two surfaces of the silver film.Coating the silver film with a photoinduced birefringence polymer film, we achieve optical modulation of the MZI output by changing the refractive index of the polymer film with a pump beam. An on/off ratio of 2. 7 is obtained for a probe wavelength of 865 nm.