Method of manufacturing mask ROM

A method of manufacturing mask ROM is disclosed. The method includes steps of (a) providing a substrate having plural gate structures thereon; (b) forming a first oxide layer for covering the substrate and the plural gate structures; (c) forming a mask layer with a ROM opening on the first oxide layer; (d) executing an implantation via the ROM opening; (e) removing the mask layer and exposing the first oxide layer; (f) forming a second oxide layer on the first oxide layer; (g) partially etching away the second oxide layer and the first oxide layer for forming a contacting opening on the surface of the substrate; and (h) forming a metal layer on the contacting opening, thereby obtaining the mask ROM. The manufacturing method can effectively prevent the gate structure from being damaged and avoid the metal line being short.