Reduced graphene oxide (RGO)/Si Schottky diode has been fabricated by a simple drop-casting/annealing process. Common combustible and/or toxic gases including CH4, O2, CO, NO2, NO, and SO2 were employed to evaluate the detection performance of such device. The relationship between current response and gas flow rate, concentration, bias voltage as well as operating time has been systematically studied, and the results indicated that the RGO/Si-based device is selective to gases like NO2 and NO. In some cases (i.e. flow rate detection), however, the current response for one gas is completely contrary to others, presumably due to the oxygen functional groups (OFGs) presiding on the surface of reduced graphene oxide. Finally, the effects of OFGs on the gas detection performance of RGO/Si-based devices were thoroughly discussed.