The instability of amorphous Si thin film transistors under uniaxial strain has been studied. Compared to the effect of tensile bias stress, larger threshold voltage (Vtha) shift is observed under compressive bias stress. These results are related to the damage of weak Si-Si bonds during the ac bias stress. However, the Vth shift of devices on the re-flattened substrate is larger after tensile strain than that of compressive strain. In addition, the defeat diminished effect of tensile situation is decreased after re-flattening the device. Therefore, after re-flattening the substrate the Vth shift resulting from tensile bias stress is larger than that of the compressive one.