Abstract: Based on the deep reactive ion etching (DRIE) processes，an important technique of micromechanical processing technology，a terahertz-band E plane 3dB four branch waveguide directional coupler at the center frequency of 0.355 THz is designed in this paper. By using the even-odd mode method and the series junctions analysis method, the structure parameters can be determined when coupling ratio is given. The simulated results by Ansoft HFSS show that in the frequency of 0.35-0.36THz, the isolation of the coupler is better than 40 dB and the return loss of each port is greater than 40dB. The lowest insertion losses are lower than 0.2dB. The designed branch waveguide directional coupler with a broad application prospect is proposed with the advantages of simple and compact structure, superior performance.