We applied a deuterium plasma treatment to the surface of polycrystalline silicon films using PECVD and observed the change with AFM, XRD, ET-IR, and SIMS measurement. A bias temperature stressing (BTS) test was carried out to evaluate the reliability of the thin-film transistors (TFT). TFTs with channel lengths as small as 2 ${mu}m$ were electrically stressed fer up to 1000 sec at room temperature. From the parameter variation such as s-factor, leakage current and on/off ratio, we suggest that the deuterium plasma treatment suppress the hot carrier effect and improve the stability of TFTs.