An atomistic quantum transport simulation is used to assess the performance of carbon nanotube (CNT) array transistors in the presence of practical non-ideal effects. We show that the non-ideal factors in the fabrication process can greatly affect the performance of the CNT array FET. The off-current can be increased by an order of magnitude when the standard deviation of tube diameter distribution is only 0.24 nm, which increases the static power consumption and degrades the on-off current ratio. A small fraction of metallic CNTs in the array can nearly sweep away the gate modulation of the current. The misalignment of the CNT orientation can both affect the on- and off-currents. The effect of electrostatic screening on electronic transport is important for device modeling and design, especially when high density packing of CNTs is required to outperform the silicon devices. To achieve an on-current of 2200 mu A/mu m, our simulation work indicates a neighboring CNT spacing below 6 nm is needed. (C) 2011 Elsevier Ltd. All rights reserved.