A method for forming an interconnect structure is provided. The method includes providing a substrate. The method also includes forming a dielectric layer on the substrate, and the dielectric layer includes an extreme low-k (ELK) dielectric layer. The method includes forming a via hole in the dielectric layer and forming a photoresist in the via hole and on the dielectric layer. The method also includes removing the photoresist by a plasma process using a CxHyOz gas and forming a conductive structure in the via hole.