A manufacturing method of semiconductor devices having metal gate includes following steps. A substrate having a first semiconductor device and a second semiconductor device formed thereon is provided. The first semiconductor device includes a first gate trench and the second semiconductor device includes a second gate trench. A first work function metal layer is formed in the first gate trench and the second gate trench. A portion of the first work function metal layer is removed from the second gate trench. A second work function metal layer is formed in the first gate trench and the second gate trench. The second work function metal layer and the first work function metal layer include the same metal material. A third work function metal layer and a gap-filling metal layer are sequentially formed in the first gate trench and the second gate trench.