A semiconductor device structure is provided. The semiconductor device structure includes a semiconductor substrate and a gate stack over the semiconductor substrate. The semiconductor device structure also includes a sealing structure over a sidewall of the gate stack, and a width ratio of the sealing structure to the gate stack is in a range from about 0.05 to about 0.7. The semiconductor device structure further includes an etch stop layer over the semiconductor substrate, the gate stack, and the sealing structure. The etch stop layer is in contact with the sealing structure.