A method for forming a semiconductor structure is provided. The method for forming the semiconductor structure includes forming a first fin structure with a first composition and a second fin structure with a second composition, oxidizing the first fin structure to form a first oxide layer and oxidizing the second fin structure to form a second oxide layer, removing the second oxide layer formed on the second fin structure, oxidizing the second fin structure to form a third oxide layer over the second fin structure, and forming a first metal gate electrode layer over the first oxide layer and a second metal gate electrode layer over the third oxide layer.