The authors investigated the effect of SiOx passivation layers on the bias stability of bottom gate amorphous (alpha-) InGaZnO4 thin film transistors (TFTs) fabricated on glass substrates. The use of rapid thermal annealing for unpassivated TFTs in air improved the device performance, showing larger drain current and field effect mobility compared to the as-fabricated TFTs. Threshold voltage (V-TH) and subthreshold gate-voltage swing (S) for both unpassivated and passivated devices were found to be nearly independent of the low-gate-voltage stress (5 V), but both were strongly affected under a relatively high-voltage stress (>10 V). The positive V-TH and S shifts after constant gate voltage stress (+20 V) of 1000 s were 1.8 V and 0.72 V decade(-1) for the unpassivated devices and 1 V and 0.42 V decade(-1) for the passivated devices, respectively. These results demonstrate that the SiOx passivation layer significantly reduced the shift in TFT's characteristics.