Abstract: The self-assembled polyelectrolyte layers were used to modify the substrate surfaces for depositing polypyrrole films via in situ polymerization and self-assembly method. The properties of the films were characterized by ultraviolet-visible spectrophotometry (UV-Vis) and atomic force microscope (AFM). The PPy films gas sensors using the interdigitated electrodes were fabricated, and the sensing properties to toxic gase NH3 at room temperature were studied. The results showed that the sensor could detect NH3 of 0~141 ppm concentrations with a high sensitivity and good response. And the effects of depositing time on the sensitivities were investigated, which showed that the best sensitivity was obtained with 20 min depositing. The sensitivities to temperature from -30℃ to +60℃ and humidity from 5.9% R.H to 59.6% R.H of the sensors were tested at last.