A manufacturing method for a semiconductor device having a metal gate is provided. First and second gate trenches are respectively formed in first and second semiconductor devices. A work-function metal layer is formed in the first and second gate trenches. A shielding layer is formed on the substrate. A first removing step is performed, so that the remaining shielding layer is at bottom of the second gate trench and fills up the first gate trench. A second removing step is performed, so that the remaining shielding layer is at bottom of the first gate trench to expose the work-function metal layer at sidewall of the first gate trench and in the second gate trench. The work-function metal layer not covered by the remaining shielding layer is removed, so that the remaining work-function metal layer is only at bottom of the first gate trench. The remaining shielding layer is removed.