A semiconductor device having a metal gate includes a substrate having a first gate trench and a second gate trench formed thereon, a gate dielectric layer respectively formed in the first gate trench and the second gate trench, a first work function metal layer formed on the gate dielectric layer in the first gate trench and the second gate trench, a second work function metal layer respectively formed in the first gate trench and the second gate trench, and a filling metal layer formed on the second work function metal layer. An opening width of the second gate trench is larger than an opening width of the first gate trench. An upper area of the second work function metal layer in the first gate trench is wider than a lower area of the second work function metal layer in the first gate trench.