Single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) with various tube diameters and constant tube length are investigated. Similar variations of on-currents, independent of the diameter, are demonstrated with the percolation theory, but off-currents significantly depend on the diameter. The temperature dependence of the off-currents attributes the observed non-negligible off-state conductance to thermally excited carriers in semiconducting SWCNTs.