Abstract: A gas sensor is provided. The gas sensor includes a substrate, a first electrode, a second electrode, a power source and a current meter. The substrate includes a planar surface, wherein the substrate is made of ion-conductive material, proton-conductive material or electron-conductive material. The first electrode is formed on the planar surface. The second electrode is formed on the planar surface, wherein a gap is formed between the first electrode and the second electrode. The power source is electrically connected to the first electrode and the second electrode, wherein the power source provides a voltage to the first electrode and the second electrode. The current meter is electrically connected to the first electrode and tie second electrode to measure a limit current passing the first electrode and the second electrode.