This letter reports the fabrication of polycrystalline silicon thin-film transistors (poly-Si TFT) on flexible plastic substrates using amorphous silicon (a-Si) precursor films by sputter deposition. The a-Si films were deposited with mixture gas of argon and helium to minimize the argon incorporation into the film. The precursor films were then laser crystallized using XeCl excimer laser irradiation and a four-mask-processed poly-Si TFTs were fabricated with fully self-aligned top gate structure. The fabrication of poly-Si TFTs on flexible polymer substrates using sputtered deposited a-Si films was discussed. It was found that the argon impurity concentration was dependent on the argon/helium mixture ratio. The precursor films were then laser crystallized using XeCl excimer laser irradiation and a four-mask-processed poly-Si TFTs were fabricated with fully self-aligned top gate structure.