The ordered mesoporous Ni-doped In₂O₃and undoped In₂O₃nanostructures have been synthesized via a nanocasting method, which is an easy, repeatable and friendly route. The structure of the as-prepared product was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption and scanning electron microscopy (SEM). The results of XRD and TEM revealed the ordered structure of undoped and Ni-doped In₂O₃. The wide-angle XRD of the samples revealed that Ni incorporation may lead to lattice deformation without destroying the original crystal structure. Gas sensors based on undoped and Ni-doped In₂O₃were fabricated and their gas sensing properties were tested. The Ni-doped In₂O₃based sensor showed excellent selectivity toward NO₂at the operating temperature of 58℃and the detection limit was 10 ppb. The response of the sensor based on mesoporous Ni-doped In₂O₃was nearly 4 times higher than that of the sensor based on mesoporous undoped In₂O₃.