We have simulated short channel carbon nanotube field-effect transistors with asymmetric source and drain contacts using a Coupled mode space approach within the non-equilibrium Green's function framework. The simulated results show that the asymmetric conduction properties under positive and negative drain-to-source voltages are caused by the asymmetric Schottky barriers to carriers at the Source and drain contacts. Under negative drain-to-source voltages. hole and electron conduction are dominated by thermionic emission and tunneling through the Schottky barrier, respectively, leading to the different Subthreshold behaviors of the hole and electron conduction. With increasing channel length, short channel effects can be suppressed effectively and ON/OFF ratio can be improved. (C) 2008 Elsevier B.V. All rights reserved.