The material characteristics of single-walled carbon nanotubes (SWCNTs) influence the performance of SWCNT thin-film transistors (TFTs). In this study, a density gradient ultracentrifugation method was used to sort surfactant (sodium deoxycholate)-dispersed SWCNTs by length. SWCNTs of 150 ± 33 nm and 500 ± 91 nm long were fabricated into TFTs. The results show that the performance of SWCNT-TFTs is tube length dependent. TFTs fabricated using 500-nm long tubes have maximum on/off ratio around 105 with the mobility at ∼0.15 cm2/(V s), which is much higher than that of TFTs using 150-nm long tubes. Shorter tubes need higher tube density to form semiconducting paths, leading to lower on/off ratio and high contact resistance. Surfactant-wrapped SWCNTs will bundle into ropes of different size when tube density is high. It is critical to control tube length as well as surfactant residue content to build high performance SWCNT-TFTs.