The degradation of n-channel poly-silicon thin film transistor (poly-Si TFT) has been investigated under dynamic voltage stress. The ON-current of TFT is 0.03 times the initial value after 1000 s stress. However, both the sub-threshold swing and threshold voltage almost kept well during the ac stress. The current crowding effect was rapidly increased with the increasing of stress duration. The creation of effective trap density in tail-states of poly-Si film is responsible for the electrical degradation of poly-Si TFT. Moreover, the damaged regions were evidenced to be mainly near the source/drain regions according to the electrical analyses.