Amorphous indium. tin oxide (ITO) thin films were grown on plastic substrates, PES (polyethersulfone) using low temperature DC magnetron sputtering. Various post-annealing techniques are attempted to optimize conductivity, transmittance, and roughness: i) conventional thermal annealing, ii) excimer laser annealing, and iii) UV irradiation. The electrical/optical properties were measured using Hall-measurement, DC 4-point resistance measurement, and UV spectrometry along with micro-structural characterization. Optimized UV treatment exhibits enhanced conductivity and smooth surface, compared to those of conventional thermal annealing and excimer laser annealing.