Analytical studies on a traveling wave direct energy converter (TWDEC) for D-3He fueled fusion are carried. out. The energy of 15MeV carried by fusion protons is too high to handle with an electrostatic device. The TWDEC controls these high energy particles on the base of the principle of a Linac. This traveling wave method is discussed and the details of proton dynamics and excitation mechanism of electric power are clarified. The TWDEC consists of a modulator and a decelerator. The applied traveling wave potential to the modulator modulates the velocity of fusion proton beams. This modulation makes a form of bunched protons at a down stream of the modulator. The decelerator has a set of meshed grids, each of which are connected to a transmission circuit. The phase velocity of excited wave on the transmission circuit is controlled as same as that of decelerated protons. The kinetic energy 15MeV of proton beams changes into an oscillating electromagnetic energy on the transmission circuit. This highly efficient direct energy converter of fusion protons brings a fusion reactor with a high plant efficiency.