Indium Gallium Zinc Oxide (IGZO) thin films were deposited onto 300 nm-thick oxidized Si substrates and glass substrates by direct current (DC) magnetron sputtering of IGZO targets at room temperature. FESEM and XRD analyses indicate that non-annealed and annealed IGZO thin films exhibit an amorphous structure. To investigate the effect of an annealing treatment, the films were thermally treated at 300 oC for 1hr in air. The IGZO TFTs structure was a bottom-gate type in which electrodes were deposited by the DC magnetron sputtering of Ti and Au targets at room temperature. The non-annealed and annealed IGZO TFTs exhibit an Ion/Ioff ratio of more than 105. The saturation mobility and threshold voltage of nonannealed IGZO TFTs was 4.92 × 10-1 cm2/V·s and 1.46 V, respectively, whereas these values for the annealed TFTs were 1.49 × 10-1 cm2/V · s and 15.43 V, respectively. It is believed that an increase in the surface roughness after an annealing treatment degrades the quality of the device. The transmittances of the IGZO thin films were approximately 80 %. These results demonstrate that IGZO thin films are suitable for use as transparent thin film transistors (TTFTs).