A shallow trench isolation (STI) and method of forming the same is provided. The STI structure comprises an upper insulating portion and a lower insulating portion, wherein the lower insulating portion includes a first insulator and an insulating layer surrounding the first insulator, the upper insulating portion includes a second insulator and a buffer layer surrounding the second insulator. A part of the buffer layer interfaces between the first insulator and the second insulator, and the outer sidewall of the buffer layer and the sidewall of the first insulator are leveled.