In this work the characteristics of p-type polysilicon thin-film transistors (poly-Si TFTs) with dynamic bias stress were investigated. The ac stress is operated with the constant drain voltage (15 V) and the varying gate voltage (0 V to -15 V) to degrade the devices. Because the self-heating effect could raise channel temperature, the Si-H bonds at the poly-Si/SiO2 interface were broken due to Joule heating. The released hydrogen reacts with SiO2 and causes the fixed charge in the gate oxide. Thus, the degradation of electrical characteristics of the device is mainly dominated by the self-heating-induced negative-bias-temperature instability effect.