In this work, we investigated a novel mechanism of polycrystalline-silicon (poly-Si) grown on tempered glass and the stability under various bias-stress effects in the performance of poly-Si thin-film transistors (TFT) fabricated on tempered glass and compared with the poly-Si TFT on bare glass. The glass shrinkage, mask misaligned width, and thermal strain of the tempered glass were 0.01 ppm, 0.9 μm, and 26.1 MPa. The compressively tensioned bare glass showed a suppression of the crystallization rate showing a small grains, whereas the tempered glass showed high crystallization rate showing a large grains. The thermal substrate stress effect on the poly-Si TFT showed serious reliability degradation, while the poly-Si TFT on tempered glass showed stable driving characteristics on the gate and hot-carrier bias stress. The better stability is due to the smooth surface and non microcracks of poly-Si on tempered glass, which is about 1.5 nm that is much smaller than that (5 nm) of a poly-Si on bare glass.