A method of chemical-mechanical polishing for forming a shallow trench isolation is disclosed. A substrate having a number of active regions, including a number of relatively large active regions and a number of relative small active regions, is provided. The method comprises the following steps. A silicon nitride layer on the substrate is formed. A number of shallow trenches are formed between the active regions one or more of which may constitute an alignment mark. An oxide layer is formed over the substrate, so that the shallow trenches are filled with the oxide layer. A partial reverse active mask is formed on the oxide layer. The partial reverse active mask exposes a portion of the oxide layer over the large active area and over the alignment mark. The oxide layer of each large active region and the alignment mark is removed. The partial reverse active mask is removed. The oxide layer is planarized.