Recently sub-picosecond electron pulses are paid attention because of valious applications of THz coherent radiation emitted from such short bunches. Although photoinjetcors using RF guns are rapidly developped as high blliiant electron sources, while thermionic RF guns are still expected to have potential ability to create high-brightness and short-pulse beams . In particular, components of a thermionic RF gun are simple, compact and low-cost than those of a photo-cathode RF gun. For creating such beams, a prototype of thermionic RF gun was designed and its characteristics have been studied by a 3-D simulation code based on an FDTD (Finite Difference Time Demain) method which has been developed so far . The gun is consists of two independently power feeding S-band RF cavities. The first cell is a cathode cell to extract the beam and the second one is an accelerating cell. This gun can be operated at modes with different RF-power ratio and phase between two RFs. A similar way of operating RF gun has been already reported by Lewellen . This paper describes the results of simulations for this thermionic RF gun.