Work on experimental and theoretical study of interference of two contra-propagating surface plasmon polaritons is reported. In the experiments, the technique of attenuated total reflection in the Kretschmann configuration is used to launch the plasmon polariton on the surface of a thin silver film (similar to 500 Angstrom in thickness), which covers the flat surface of a semicylindrical prism of optical glass. A mirror is used to send back the reflected light beam to the same area of the film. The reflected beam from the mirror launches another surface plasmon polariton in the direction opposite to the first surface plasmon polariton. Light beam from a He-Ne laser is used in the experiments. Noncoherent light from a monochromator is also used to help alignment. An interference pattern of the two surface plasmon polaritons is observed and is recorded with an optical microscope located in the air side near the film. The above device is also described theoretically with numerical calculations of interference fringe spacing.