The influence of grain boundary (GB) on stability of poly-silicon thin film transistor (TFT) have been investigated in this work. The work was supported by the National Science Council of the Republic of China and AUO. We used ac stress and dc stress conditions to stress different TFTs, and investigate the influence of grain boundary by use of electrical analysis. The SLS poly-Si TFT which does contain GB perpendicular to the channel direction owns the higher ability against dc stress and poorer ability against ac stress than the poly-Si TFT which does not contain GB. The physical mechanism for these results has been reasonably deduced by use of TFT device simulation tool (ise-tcad).