We discuss here the influence of oxide semiconductor thickness on thin-film transistor (TFT) characteristics. We have determined this influence by measuring the transfer characteristics of amorphous InGaZnO (IGZO) TFTs having various IGZO thicknesses and using a simple method to calculate the depletion width in IGZO films. ON current was nearly constant with respect to IGZO thickness because it depended on a high electron density in an accumulation region sufficiently thinner than the IGZO film. The threshold voltage shifted negatively with increasing IGZO thickness, which indicates that a thicker IGZO film requires a higher negative gate voltage for it to be fully depleted. Calculation results suggest that threshold voltage variation due to IGZO thickness variation increases with increasing donor density and IGZO thickness.