A semiconductor structure and a manufacturing method thereof are disclosed. The semiconductor structure includes an isolation layer, a gate dielectric layer, a first work function metal, a first bottom barrier layer, a second work function metal, and a first top barrier layer. The isolation layer is formed on a substrate and has a first gate trench. The gate dielectric layer is formed in the first gate trench. The first work function metal is formed on the gate dielectric layer in the first gate trench. The first bottom barrier layer is formed on the first work function metal. The second work function metal is formed on the first bottom barrier layer. The first top barrier layer is formed on the second work function metal.