Terahertz (THz) surface plasmon generation via nonlinear mixing of laser eigenmodes in an optical fiber coated with ultrathin metal foil and possessing a dielectric constant ripple of wave number q is investigated. The fiber supports THz surface plasma wave (SPW) with plasmon resonance in the THz domain and is controllable by film thickness. The lasers exert a difference frequency ponderomotive force on the electrons of the metal film. The ponderomotive force induces a nonlinear current driving the difference frequency THz SPW. THz amplitude to laser amplitude ratio of the order ∼10(-2) can be achieved by CO2 laser in megawatt range.