By using the electromagnetic particle-in-cell(PIC) MAGIC code, the process of field emission is simulated. The simulation includes two representative field emission devices(Spindt-type field emitter and diamond film field emitter). For Spindt-type field emitter, the relation between the characteristics and the radius of curvature of the emitter tip , the relative height between the emitter tip and the gate are studied. As regards the diamond film field emitter, the characteristics of field emission microtriodes and microtetrodes are compared with each other and the influence of the area of diamond film is investigated. The resul ts show that the characteristics of diamond film field emitter has an advantage over Spindt-type field emitter.