We experimentally achieved optical third harmonic generation (THG) from two-dimensional periodically arrayed gold-island films. The gold-island films were fabricated using micro-spherical lithography. By using a reflection-type optical system, these arrays can produce significant THG with the help of a spectrum-resolved femto-second laser. At the localized surface plasmon (LSP) excitation wavelength of the fundamental wave, THG is greatly enhanced due to the strong light localization effect. The radiation direction of THG satisfies the momentum matching condition.