The high sensitivity and selectivity of rGO-SnO2 nanocomposites for NH3 at room temperature were prepared by precipitation-roasting method. X-ray diffraction (XRD), Fourier infrared Spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and specific surface area (BET) were used to characterize and analyze pure SnO2 with rGO (1.0%)-SnO2 nanocomposites. Compared with the pure SnO2, the crystal size of SnO2 in the rGO (1.0%)-SnO2 nanocomposites is smaller, about 6 ~ 20 nm, with a larger specific surface area of 33m2/g. The sensitivity of rGO (1.0%)-SnO2 nanocomposites to 0.01% NH3 reached 49.6%, which was 2.1 times of that of pure SnO2 at the same NH3 concentration, and the response and recovery time were 21 s and 204 s, respectively, which was 24 s and 10 s shorter than that of pure SnO2, with good repeatability, selectivity and stability. The excellent gas sensing properties of rGO (1.0%)-SnO2 nanocomposites can be attributed to the interaction of p-n heterojunction produced by rGO and SnO2 and dissolved NH3 ionized conductive ions.