An ultrathin metalfilm deposited on dielectric plate supports low loss THz surface plasma wave (SPW). The SPW field falls off away from the metalfilm both inside the dielectric as well as in free space. Two lasers normally incident, from free space on the planar structure, exert a difference frequency ponderomotive force on the free electrons of the film and resonantly excite THz SPW. The ratio of SPW amplitude to lasers is 10−3 at laser intensity of 3 × 1012 W/cm2 at wavelength. The growth rate falls monotonically because at higher frequency the coupling of SPW is weak.