Analyzing volatile organic compounds in exhaled breath is a noninvasive method that has potential for disease assessment. Herein, acetone sensors based on porous In₂O₃flowers are synthesized through a facile solvothermal method. Structural characterizations demonstrate that the three-dimensional In₂O₃flower-like structure comprises porous ultrathin two-dimensional nanosheets. The structural porosity results in a large specific surface area and fast gas diffusion, which can improve the gas sensing performance. The crystalline structure of the prepared hierarchical flower-like In₂O₃is a mixed phase of rhombohedral and cubic phases according to the XRD patterns. The sensing properties of the cubic-rhombohedral In₂O₃(bcc-rh-In₂O₃) are also investigated. Compared with commercial-In₂O₃-based sensor, the bcc-rh-In₂O₃sensor exhibits 2.9 times higher response (Ra/Rg= 12) and shorter response time (ca. 2 s) toward 50 ppm acetone. The bcc-rh-In₂O₃sensor has a low detection limit of 10 ppb in atmosphere and can easily distinguish acetone from ethanol. The extraordinary properties of the bccrh- In₂O₃sensor are mainly attributed to the n-n heterojunction between rh-In₂O₃and bcc-In₂O₃.