Ag doped ZnO film, a new kind of gas sensing optical thin film, was deposited by reactive rf magnetron sputtering technique as well as pure ZnO thin film. The transmission spectra to NO x of these films were measured, then the variation of sensitivity was derived from the transmission spectra. It was found that the sensitivity to NO x of Ag doped ZnO films was higher than that of pure ZnO films. The best sensitivity to NO x was achieved with a 5% Ag doped ZnO thin film, which was examined by auger electron spectroscopy (AES). The gas sensing transmission properties were analyzed by considering the Langmuir type adsorption equilibrium relation and it was shown that the theoretic analyses were excellently coincide with the experimental results.