In today’s semiconductor industry, transistor size has been continuously scaled down due to the competition between developers to provide a better performance device. Based on Moore’s Law, the use of metal oxide semiconductor field-effect transistor (MOSFET) technology might end due to its dimensional limitation, which affects its performance. Carbon nanotube field-effect transistor (CNTFET) has become a good candidate to replace MOSFET technology due to its carbon nanotube properties (CNT). In the CNTFET design parameters, the changes in the diameter of CNT and the dielectric materials of the oxide layer significantly affect the transistor’s performance. The results show that by increasing the diameter of CNT and having a higher dielectric constant material, the on-current (Ion) and the transconductance, gm of the CNTFET will significantly increase. This effect will produce a device with a higher current ratio (Ion/Ioff) and provide a better device performance. The study also included the effect of this design parameter on the channel’s average electron velocity. From this study, it can be deduced that the diameter of CNT and the dielectric material of the oxide layer greatly affect the transistor’s performance.