The subthreshold characteristics of both hydrogenated and unhydrogenated sub-micrometer polysilicon thin-film transistors have been investigated in detail, The subthreshold slope of unhydrogenated TFTs becomes steeper at higher drain voltage. This is attributed to the floating-body effect that results from the positive feedback between transistor current and impact ionization current. The floating-body effect of poly-TFTs was found to relate to grain size, drain voltage and hydrogenation. For the TFTs with larger poly-grain, the floating-body effect was more obvious and appeared at lower drain bias. After hydrogenation, the floating-body effect was suppressed. Therefore, the subthreshold swing of hydrogenated TFTs is continuously increasing with drain voltage.