We have fabricated organic field effect transistors (OFETs) with liquid crystalline organic semiconductor, 2-decyl-7-phenyl-benzothienobenzothiophene (Ph-BTBT-10) and mixing a small amount of p-type dopant, 1,3,4,5,7,8-hexafluorotetracyanonaphthoquinodimethane (F6-TCNNQ) to control the threshold voltage. At low voltage below 4 V, Ph-BTBT-10 OFETs showed threshold voltage (Vth) around 0V, subthreshold swing (SS) of 100~200 mV/dec, On/Off ratio of 10⁶, and mobility of 2~3 cm²/Vs. Vth was shifted to positive from -0.5 V to 0.5 V depending on doping concentration due to the increase in hole carrier in the Ph-BTBT-10 channel by hole doping.