A gas sensor is provided. The substrate of the gas sensor has a first surface, a second surface and a cavity. The cavity has an opening at the first surface. An insulating film is disposed on the first surface and covers the opening. A heating unit is embedded in the insulating film and located above the opening. An electrode pair is disposed on the insulating film and electrically separated from the heating unit. A buffer layer is disposed on the insulating film and located above the heating unit. The buffer layer is electrically connected to the electrode pair, and at least part of an orthogonal projection of the buffer layer on the first surface is located on the substrate next to the opening. The gas sensing layer is disposed on the buffer layer and has a nano-catalyst therein.