Poly(3, 4-ethylenedioxythiophene) (PEDOT) has applications in many areas due to its exciting electrical performance and high stability. Since it has very low thermal conductivity, it is also a good organic thermoelectric material. However, the ZT value of pure PEDOT is rather low, because the electrical properties such as conductivity are still not satisfactory. It is found that the thermoelectric performance can be enhanced by adding inorganic thermoelectric materials into PEDOT to form composites. In this paper, we synthesize a composite of In2O3/PEDOT by chemical oxidation. Microstructure of the composite is studied by X-ray diffraction, infrared spectroscopy, transmission electron microscope, and positron annihilation spectroscopy. The XRD measurements show that the pure PEDOT sample is amorphous, and the crystallinity in composite sample is contributed by In2O3. Besides, the diffraction peaks become sharper with increasing the In2O3 content. Transmission electron microscope measurements confirm that the PEDOT sample is amorphous and the shapes of In2O3 particles are regular. The surfaces of the In2O3 particles are wholly coated with thin layers of PEDOT, and when the In2O3 content is higher than 22 wt%, the In2O3 particles cannot be uniformly dispersed in pure PEDOT layers. The positron annihilation measurements reveal the interface structure in the In2O3/PEDOT composite, which can capture positron and cause the lifetime of positron to increase. The relative quantity of interface increases with In2O3 content increasing. However, when the In2O3 content is more than 22 wt%, the interface structure is destroyed. All the measurements show that when the In2O3 content is lower than 22 wt%, the In2O3 nanoparticles are well dispersed in PEDOT. The electrical conductivity of In2O3/PEDOT composite increases with In2O3 content increasing. At room temperature, the electrical conductivity of PEDOT is 7.5 S/m, while in the In2O3/PEDOT sample with 12.3 wt% In2O3, a maximum electrical conductivity of 25.75 S/m is obtained. When the In2O3 content increases from 0 to 22 wt%, the power factor of the composite increases rapidly from 14.5×10-4 to 68.8×10-4 μW/m·K2. On the contrary, the thermal conductivity shows decrease compared with the thermal conductivity of pure PEDOT. The ZT value of the composite increases from 0.015×10-4 to 0.073×10-4. Our results indicate that the thermoelectric properties of In2O3/PEDOT composite can be effectively improved compared with those of the pure PEDOT