Ag-doped hollow urchin-like spheres In₂O₃hierarchical nanostructures are developed for NO₂detection. Such unique architectures are synthesized by a facile and efficient solvothermal route combined with the subsequent thermal treatment. Various techniques, including X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were employed to acquire the crystalline and morphological information of the as-obtained samples. Gas sensing performances of the sensor devices fabricated from pure and Ag-doped In₂O₃were systematically investigated. The results indicate that the sensors based on Ag-doped In₂O₃in certain molar ratio of AgNO₃to In₂O₃(1:100) exhibit the largest response toward 1 ppm NO₂, which is almost 23 times higher than that of the sensor based on pure In₂O₃at the optimum operating temperature. It demonstrates that the Ag-doping can significantly improve the response to NO₂. The excellent and enhanced NO₂sensing performances of Ag-doped In₂O₃can be attributed to its novel hierarchical structure and the catalytic activity of Ag nanoparticles.