Polyaniline thin layers were successfully coated on the surface of graphene sheets through in situ chemical oxidative polymerization. By integrating into a resistive sensor design, the sensing properties of the nanocomposites were evaluated. It is found that the nanocomposite sensors exhibited high sensitivity with fast response and recovery time, and good reproducibility charactics for ammonia gas at room temperature even exposed to low concentration of 10 ppm, implying that the nanocomposite could be a promising candidate as a gas sensor for ammonia gas.