We investigated the electrical properties of solution-processed zinc oxide (ZnO) thin-film transistors (TFTs) with varying the size of ZnO crystallites in the film. The TFTs having the bead-shaped ZnO crystallites with the average diameter of 2 mu m apparently operated in depletion mode, whereas the devices exhibited an accumulation operation when the crystallite size of ZnO reduced to sub-micron range. Considering the difference between bulk and channel electrical conductivities in solution-processed ZnO TFTs, these results are explained with the interplay between the crystallite size of ZnO and the electrical conduction in the TFT.