The metal-conducting single-walled carbon nanotubes (m-SWNTs) with small diameters (0.7 nm-1.1 nm) are selectively removed from the single-walled carbon nanotubes (SWNTs) by using HNO₃/H₂SO₄ mixed solution. Semiconducting singlewalled carbon nanotubes (s-SWNTs) can be separated efficiently from the SWNTs with high controllability and purity based on this novel method, and the outcome is characterized by Raman spectrum. Moreover, the organic field effect transistors (OFETs) are fabricated based on the poly (3-hexylthiophene-2, 5-diyl) (P3HT), and untreated SWNTs and separated SWNTs (s-SWNTs) are mixed with P3HT, respectively. It could be found that the P3HT/s-SWNT device exhibits a better field effect characteristic compared with the P3HT device. The current on/off ratio is increased by 4 times, the threshold voltage is also increased from -28 V to -22 V, and the mobility is increased from 3×10^-3 cm²/Vs to 5×10^-3cm²/Vs.