In this report, NH3 gas sensors based on Ag-doped gamma-Fe2O3/SiO2 composite films are investigated. The composite films were prepared with a sol gel process, and the films' electrical resistance responded to the change of NH3 concentration in the environment. The SEM and AFM investigations showed that the films had a porous structure, and the XRD investigation indicated that the size of Ag particles changed with the modification of Ag loading content. Through a comparative gas sensing study among the Ag-doped composite films, undoped composite film, gamma-Fe2O3 film, and SiO2 film, the Ag-doped composite films were found to be much more sensitive than the sensors based on the undoped composite film and gamma-Fe2O3 film at room temperature, indicating the significant influences of the SiO2 and Ag on the sensing property. Moreover, the sensor based on Ag-doped (4%) gamma-Fe2O3/SiO2 composite film was able to detect the NH3 gas at ppb level. Conversely, the responses of the sensor to other test gases (C2H5OH, CO, H-2, CH4 and H2S) were all markedly low, suggesting excellent selectivity. (C) 2015 Published by Elsevier B.V.