Virtual cathode oscillators (vircators) are pulsed, high-power microwave generation devices with the advantages of configuration simplicity and wide frequency-tuning ability. Owing to the ability of electron beam pre-modulated, the field emission cathode is often used as electron sources n vircators of nowadays. Some emitted current density models for the field emission had been proposed, e.g., Fowler-Nordheim equation for microscopic flat plate diode [I], and relativistic Child-Langmuir equation for diode with space charge limited effect . But it was seldom discussed which model is more suitable when the emission current is so large as in vircators [l]. In this paper, we will investigate the characteristics of the vircators with field emission cathode via computer simulation.