A simulation of the carbon nanotube network (CNN) based FET device is proposed. The device structure has two concentric electrodes used as source and drain, which are connected by a CNN as a semiconductor channel layer and merged into aqueous solution for sensor application. The simulation system is based on the transport equation in the CNN, the Poisson equation in the insulator, CNN, and aqueous solution with appropriate auxiliary equations for carrier statistics in the various regions. Current versus voltage relationships were obtained and compared with the measurements. The new phenomenon, hereafter called the self gating effect, has been observed.