The response of a long-period grating coated with the sol-gel derived films on the cladding of the fiber grating region to surrounding medium (gases) was studied in this paper based on coupled-mode theory. A four-layered numerical model is also developed to determine the dependence of the central wavelengths of the attenuation bands on the thin film optical parameters ( thickness d3 and refractive index n3). By analyzing the relation between the sensitivity Sn and the thin film optical parameters and the fiber grating parameters (the grating period, the core index change and the grating length ), the optimal optical parameters of thin film layer of the sensor is obtained. Data simulation shows that the sensitivity of this scheme to refractive index of the films is predicted to be more than 107. In the end a gas-sensitive long-period grating sensor is fabricated according to the theoretical optimization results. The preliminary gas-sensing experiment was performed, and a novel LPFG gas sensor special to C2H5OH and CH4 was established.