A manufacturing method of a semiconductor device includes the following steps. A first transistor is formed on a substrate. The first transistor includes a first semiconductor channel structure and two first source/drain structures. The first semiconductor channel structure includes first horizontal portions and a first vertical portion. The first horizontal portions are stacked in a vertical direction and separated from one another. Each of the first horizontal portions is elongated in a horizontal direction. The first vertical portion is elongated in the vertical direction and connected with the first horizontal portions. The two first source/drain structures are disposed at two opposite sides of each of the first horizontal portions in the horizontal direction respectively. The two first source/drain structures are connected with the first horizontal portions. A top surface of the first vertical portion in and a top surface of one of the first horizontal portions are coplanar.