A semiconductor integrated circuit includes a substrate, a multi-gate transistor device positioned on the substrate, and an LDMOS device positioned on the substrate. The substrate includes a plurality of first isolation structures and a plurality of second isolation structures. A depth of the first isolation structures is smaller than a depth of the second isolation structures. The multi-gate transistor device includes a plurality of first fin structures and a first gate electrode. The first fin structures are parallel with each other and spaced apart from each other by the first isolation structures. The first gate electrode is intersectionally arranged with the first fin structures, and covers a portion of each first fin structure. The LDMOS device includes a second gate electrode covering on the substrate. The LDMOS device is electrically isolated from the multi-gate transistor device by another second isolation structure.