A computational model for studying the metal and nanotube interface layer properties in Carbon nanotube field effect transistors (CNT-FETs) has been carried out. The CNT-FETs can be fabricated both with Ohmic and Schottky contacts. Here we have focused on Schottky barrier which operate bymodulating the transmission coefficient of carriers through the Schottky barrier. The behavior of the devices has been studied by using Landauer-Buttiker formalism. Finally the variation of current versus channel properties, voltage and other properties has been calculated via our model. The ambipolar behavior was explained based on the Schottky-barrier-controlled transistor model, where the transistor action occurs primarily by changing the Schottky contact resistance by the gate voltage. The calculation results show a fair agreement with other theoretical and experimental results.