The present invention provides a structure of a gas sensor, comprising: a support, having a front side, a back side opposite to the front side, a cell region, and a peripheral region circling the cell region; a cavity, formed on the back side of the support in the cell region; a heater, disposed on the front side of the support covering the cavity; a sensing element, disposed on the heater; and a sealing layer, formed on the back side of the support covering inside the cavity.