Out of hundreds of as-fabricated back-gated carbon nanotube field effect transistors we made, five devices which exhibit indiscernible hysteresis are found. The cause of these hysteresis-free devices is investigated based on the energy band and interface trap theory, and it is attributed to the bundling effect of single-walled carbon nanotubes which will result in a reduced bandgap in the single-walled carbon nanotube bundle compared with an isolated semiconducting carbon nanotube. A specific requirement of the SWNT bundle to satisfy the presumed explanation is also proposed.