This paper deals with different parametric effects to analyses the behaviour of carbon nanotube field effect transistors (CNTFETs) under non-ballistic conditions and based on the changes of gate dielectric constant the performance of CNTFETs has been explored in detail as functions of different parameters such as temperature and gate oxide thickness. A thorough study of the combined non-ballistic effect on the performance of CNTFETs has been conducted with different principle characteristics of CNTFETs and the output of the device has been analyzed. The on/off current ratio decreases with the decreasing value of the gate insulator thickness under non-ballistic regime. With higher thickness of oxide and dielectric constants, quantum capacitance of CNTFET decreases despite of non-ballistic movements. This effect continues with the total capacitance of the device. The decrement of the ratio of transconductance to drain current with respect to temperature indicates the increase in gate capacitance with high dielectric constants value and oxide thickness. Under higher value of temperature and dielectric constants, current ratio degrades considerably.