We report on the effects of He plasma treatment for reducing contact resistances of ZnO thin-film transistors (TFTs). A TFT with Ti (20 nm)/Au (40 nm) ohmic contacts was fabricated by exposing the ohmic contact area to He plasma before the formation of the Ti/Au ohmic contacts. Schottky like behavior was turned into ohmic behavior by the He plasma treatment, indicating the effectiveness of the treatment for reducing the contact resistance for ZnO TFTs. Interestingly, the sheet resistance of the ZnO channel layer decreased from 53.8 GΩ/sq to 6.41 GΩ/sq although the channel portion of ZnO was not treated by the He plasma. The transconductance for a 20-µm-gate TFT was also improved from 4.6 × 10−6 S/mm to 1.2 × 10−4 S/mm.