We propose underwater laser annealing (WLA) for the inactivation of electrical defects in polycrystalline silicon thin-film transistors (poly-Si TFTs) at super low-temperature. This technique can reduce the temperature of inactivation process drastically, and it requires only UV laser and deionized water. We performed WLA after the fabrication of top-gate type poly-Si TFTs. After WLA, the field-effect mobility of poly-Si TFTs increased from 52 to 72 cm. The TFT surface was exposed to water vapor which was generated by laser irradiation, resulting that electrical defects were inactivated by active species in water vapor.