The present disclosure provides a semiconductor structure. The semiconductor structure includes a fin structure on a substrate; a first gate stack and a second gate stack formed on the fin structure; a dielectric material layer disposed on the first and second gate stacks, wherein the dielectric layer includes a first portion disposed on a sidewall of the first gate stack with a first thickness and a second portion disposed on a sidewall of the second gate stack with a second thickness greater than the first thickness; a first gate spacer disposed on the first portion of the dielectric material layer; and a second gate spacer disposed on the second portion of the dielectric material layer.