Indium oxide/polyaniline (In2O3/PANI) composite nanofibers with the solid or hollow structure of In2O3 were successfully synthesized via a facile and efficient combined method of electrospinning, high-temperature calcination, and in situ polymerization. Various techniques, including scanning electron microscopy, Brunauer-Emmett-Teller, Fourier transform infrared, transmission electron microscopy, and X-ray diffraction, were employed to acquire the morphological, structural, and crystalline information of the prepared samples. The gas sensing performances of the as-obtained composite nanofibers were investigated by a home-made gas sensing test system at room temperature. All the results demonstrated that both solid and hollow In2O3/PANI composite nanofibers had higher response values than those of pure PANI sensor. In addition, the ammonia sensing properties of hollow In2O3/PANI composite nanofibers were much better than that of the solid ones. Furthermore, hollow In2O3/PANI composite nanofibers also showed ideal repeatability and selectivity due to their more and different types of P-N heterojunctions.