A semiconductor device structure and the fabrication method are provided. The semiconductor device structure includes a first channel structure and a second channel structure over a substrate. The second channel structure is longer than the first channel structure. The semiconductor device structure also includes a first gate stack over the first channel structure, and the first gate stack has a first width. The semiconductor device structure further includes a first gate spacer extending along a sidewall of the first gate stack. In addition, the semiconductor device structure includes a second gate stack over the second channel structure and a second gate spacer extending along a sidewall of the second gate stack. The second gate stack has a portion extending along the second gate spacer, and the portion of the second gate stack has a second width. Half of the first width is greater than the second width.