The back‐channel‐etched (BCE) amorphous InGaZnO thin‐film transistors with different channel widths (Ws) are fabricated. The performance of VTH depends on both channel width (W) and drain voltage (VD) in this work. It is shown that neither W nor VD can create influence in VTH when W or VD is relatively small. However, when both W and VD are large enough, there will be an anomalous phenomenon that VTH increases with the increasing W or the increasing VD. The self‐heating effect can be used to account for this anomalous dependence of VTH on W and VD.