Si-doped GaAs nanowires (NWs) were grown on (111)Si substrate by MBE-VLS method. The electrical characteristics of the GaAs NWs were measured. A joule heater was arranged near the tip of NW for making the gradient of substrate temperature. The obtained Seebeck coefficient of the GaAs NW increases linearly with a rise in temperature. The thermoelectric power of the Si doped GaAs NW was determined by the hole diffusion. It was estimated that the hole density in the Si-doped GaAs NW at room temperature was 5.9×1018 cm-3 from the slope of the temperature dependence of the Seebeck coefficient in the Si-doped GaAs NW. At room temperature, the Seebeck coefficient, thermoelectric power factor, and thermoelectric figure of merit (ZT) were 429 μV/K, 271μW/mK2, and 1.5×10-3, respectively.