A new model to study the gas detection mechanism of carbon nanotube (CNT) based ionization gas sensor has been developed. The model incorporates electron field emission property of the CNTs. The CNT density and aspect ratio variations which are major factors involved in the field emission are also included. The CNT field emission is controlled with density and aspect ratio parameters in the model. The model is then embedded in the standard Particle-In-Cell/Monte-Carlo- Collision (PIC-MCC) codes. With the help of these enhanced PIC-MCC codes the simulation of gas breakdown behaviour with CNT field emission and density variation effects become possible for the first time. From the simulations, it is observed that the electrostatic screening effects are reduced and the field emission is high for the moderate CNT density on the electrode. © 2011 IEEE.