We investigated the effects of oxygen in amorphous silicon (a-Si) and polycrystalline silicon (poly-Si) thin films using excimer laser annealing (ELA). The dehydrogenation process prior to ELA was performed in air and N2 ambient, which was the method controlled oxygen content in a-Si thin films. We observed that the thin film of oxygen controlled process had lower oxygen concentration and oxidized silicon than those of air ambient annealing. It was improved the quality of poly-Si thin film and field effect mobility of poly-Si TFTs.