The experiments on the GaAs/GaAlAs traveling wave directional coupler optical modulator are done. With aid of a system for measuring the microwave chararcteristics of the traveling wave electrode. Then the paramenter of the modified structure of the device are determined and the traveling wave optical modulator is fabricated and measured. The coplanar traveling wave electrode is used in this device and the measured breakdown voltage is 28 V. Under the measure system with 1.06 μm YAG laser, the switch voltage of 8.5 V is obtained. The velocity mismatching between the microwave, measured by the network analyzer, and the lightwave in the waveguide of the device is less than 3% and the microwave transmission loss is less than 10 dB/cm at 30 GHz which indicates the 3 dB optical modulation bandwidth of the traveling——wave modulator is greater than 32 GHz.