System analysis has been carried out for a finding of key design parameters of future DEMO. The tokamak plasma parameters, which determine plant core performance and engineering requirements are searched. In this analysis, the plasma configuration based on ITER along with several physical and engineering constraints, such as plasma shape and plasma current, are given as an input. With this given constraints, parameters such as major radius, normalized plasma beta, toroidal magnetic field are investigated to look for a design of DEMO plant. These analysis results also would be used as a guideline for the possible design of the magnet and blanket.