This work presents a plasmon-enhanced photo-conductive antenna for high power THz emission using the localized surface plasmon resonance of hierarchically patterned silver nanoislands. The silver nanoislands on a photoconductive region enhance the electrical amplitude of pump laser beam and the scattering of the pump beam so that the optical power transmitted toward the substrate is amplified. As a result, a high transition rate of the photo-excited carrier was obtained. The plasmon-enhanced antenna shows 1.6-fold increase of peak-to-peak amplitude of the emitted THz wave compared to the photo-conductive antenna without silver nanoislands.