To apply the Power MOSFET to the high powerd circuits, the parallel operation of the Power MOSFET must be considered because of their low power rating. This means, in practical applications, design methods for the parallel operations are required. However, it is very difficult to investigate the problem of parallel operations by directly changing the internal parameters of the MOSFET. Thus, in this paper, the effects of internal parameters for the parallel operation are investigated using SPICE program which is often used and known that the program is very reliable. The investigation results show that while the gate resistance and gate capacitances are the parameters which affect to the dynamic switching operations, the drain and source resistances are the parameters which affect to the steady-state current unbalances. Through this investigation, the design methods for the parallel operation of the MOSFET are suggested, which, in turn, contributes to the practical use of Power MOSFETs.