A semiconductor device structure is provided. The semiconductor device structure includes a semiconductor substrate including a first well region and a second well region that have different conductivity types and are adjacent to each other. A first fin structure protrudes from the semiconductor substrate and is formed in the first well region. A second fin structure protrudes from the semiconductor substrate and is formed in the second well region and adjacent to the first fin structure. A first multi-step isolation structure that includes a first isolation portion is formed between the first fin structure and the second fin structure. A second isolation portion extends from the bottom surface of the first isolation portion. The second isolation portion has a top width that is narrower than the bottom width of the first isolation portion.