An air gap fabricating method is provided. A patterned sacrificial layer is formed over a substrate, and the material of the patterned sacrificial layer includes a germanium-antimony-tellurium alloy. A dielectric layer is formed on the patterned sacrificial layer. A reactant is provided to react with the patterned sacrificial layer and the patterned sacrificial layer is removed to form a structure with an air gap disposed at the original position of the patterned sacrificial layer.