As for the modulation process of a traveling wave direct energy converter (TWDEC), which will be used in D-3He fusion power generation and has a principle of a reversal process of linear accelerator, numerical analysis of particle orbits has been performed. Space charge effect is taken into account to examine lower modulation effect found in simulation experiments. In the high incident particle density expected in the simulation experiments, the influences of space charge effect on modulation process have been confirmed in the view points of radial spread, axial velocity, and bunching characteristics.