Crystalline phase regulation could optimize the band gap, which has a great impact on the amount of chemisorbed gas molecules on the gas sensing materials. Herein, a facile route of hydrothermal method followed by calcination treatment was used to synthesize cubic bixbyite-type (C-In2O3), rhombohedral corundum-type (Rh-In2O3) and the mixed phase In2O3 (Rh+C-In2O3). The band gap of C-In2O3 was narrowed to a suitable value (2.38 eV) and the relative percentage of chemisorbed oxygen was enhanced (31.8%). The sensing results to ozone (O3) indicated that the C-type structure stood out. The gas sensor based on C-In2O3 exhibited extraordinary O3 sensing performances with a response of 5.7 (100 ppb) and an ultralow limit of detection of 30 ppb. The amazing results could be attributed to the narrow band gap and the enrichment of chemisorbed oxygen. This work inspires a new perspective to design highly sensitive and reliable O3 sensors.