Based on the 0.1 μm GaAs HEMT process, a miniature directional coupler chip operating in 0.1-0.14 THz is designed. The equivalent electric length of the transmission line is increased by loading the open branch to miniaturize circuit structure. The physical size of the coupler is further reduced by using the zigzag line，which is realized by the open branch and node line. Simulation results show that the designed miniaturized directional coupling chip has a center operating frequency of 0.13 THz, a relative bandwidth of more than 30%, an in-band echo loss of more than 20 dB, an in-band insertion loss of less than 1 dB, a coupling degree of (10±0.5) dB, a phase difference of (90°±3.5°), and a size of 0.21 mm×0.19 mm(without pad).