Brush-like hierarchical In 2 O 3 nanostructures on SnO 2 nanowires have been synthesized by a two-step thermal vapor transport process. To elucidate the growth mechanism, the structural evolution of hierarchical In 2 O 3 nanostructures during the synthesis process was carefully examined. The experimental results showed that a self-catalytic vapor-liquid-solid (VLS) growth mechanism was responsible for the growth of hierarchical In 2 O 3 nanostructures. Compared with the film counterparts, the hierarchical In 2 O 3 nanostructures exhibited a higher sensitivity to low-concentration CO, which can be attributed to more active centers on the surface obtained from the enhanced oxygen vacancy defects and formation of bridging point contacts in the random network between the two electrodes.