InGaZnO (IGZO) thin-film transistors (TFTs) modified by self-assembled monolayers (SAMs) based on triethoxysilane (TES) with three different alkyl chain lengths were fabricated and the relationship between the SAM chain length and the TFT electrical properties was investigated. The mobility increased and the hysteresis of transfer curves was reduced after SAM-modification, owing to less adsorption/ desorption effect on the IGZO surface. IGZO-TFTs modified by TES with longer alkyl chain lengths exhibited better electrical performance, which was attributed to the formation of a more hydrophobic and higher ordered monolayer due to the cohesive interaction between the molecules.