Various indium oxide nanostructures of 1D-wires, 2D-hexagonal plates, 3D-cubes and donuts were synthesized, and their fundamental characteristics and CO oxidation activities were studied in detail. X-ray diffraction and Raman analysis revealed that the as-synthesized wires and cubes are orthorhombic InOOH and cubic In(OH)₃, respectively. Upon annealing at 700 °C in air, all the as-prepared samples were recrystallized to cubic In₂O₃. The direct band gap of various as-synthesized nanostructures was estimated to be ∼2.7 eV from the UV-Vis absorption. Two broad photoluminescence peaks were found at 360 and 450 nm, which are attributed to the oxygen vacancies. The CO oxidation activities were in the order of hexagonal plates ≤ donuts < cubes < wires, tested by temperature-programmed reaction mass spectrometry. The difference in activity is explained on the basis of the surface area and oxygen vacancies of different nanostructures. In particular, the wires showed the CO oxidation onset at around 320 °C, which is 280 °C lower than that of hexagonal plates. The detailed morphology dependent properties and CO oxidation activities of various In₂O₃ nanostructures presented in this study provide new insights into sensor, energy, and environmental applications.