A manufacturing method of a semiconductor device includes the following steps. A barrier layer is formed in a first region and a second region of a semiconductor substrate. The barrier layer formed in the first region is thinned before a step of forming a first work function layer on the barrier layer. The first work function layer formed on the first region is then removed. The process of thinning the barrier layer in the first region and the process of removing the first work function layer in the first region are performed separately for ensuring the coverage of the first work function layer in the second region. The electrical performance of the semiconductor device and the uniformity of the electrical performance of the semiconductor device may be improved accordingly.