High-performance amorphous (α-) InGaZnO-based thin film transistors (TFTs) were fabricated on flexible polyethylene terephthalate substrates coated with indium oxide (In2O3) films. The InGaZnO films were deposited by rf magnetron sputtering with the presence of O2 at room temperature. The n-type carrier concentration of InGaZnO film was ∼2×1017cm-3. The bottom-gate-type TFTs with SiO2 or SiNx gate dielectric operated in enhancement mode with good electrical characteristics: saturation mobility 11.5cm2 V-1 s-1 for SiO2 and 12.1cm2 V-1 s-1 for SiNx gate dielectrics and drain current on-to-off ratio ≫105. TFTs with SiNx gate dielectric exhibited better performance than those with SiO2. This is attributed to the relatively high dielectric constant (i.e., high-k material) of SiNx. After more than 500 h aging time at room temperature, the saturation mobility of the TFTs with SiO2 gate dielectric was comparable to the as-fabricated value and the threshold voltage shift was 150 mV.