A gas sensor is provided. The gas sensor includes a post, an ion conductive layer, a first sensing portion and a second sensing portion. The post includes a first end, a second end, a side surface and a groove, wherein the groove is formed on the side surface of the post, and an opening is formed on the first end connecting with the groove. The ion conductive layer is formed on the side surface of the post, including a first surface and a second surface, wherein the first surface is opposite to the second surface, and the ion conductive layer and the groove compose a chamber. The first sensing portion is formed on the first surface. The second sensing portion is formed on the second surface corresponding to the first sensing portion, wherein the first sensing portion is located in the chamber.