A semiconductor device includes first and second Fin FET and a separation plug made of an insulating material and disposed between the first and second Fin FETs. The first Fin FET includes a first fin structure extending in a first direction, a first gate dielectric formed over the first fin structure and a first gate electrode formed over the first gate dielectric and extending a second direction perpendicular to the first direction. The second Fin FET includes a second fin structure, a second gate dielectric formed over the second fin structure and a second gate electrode formed over the first gate dielectric and extending the second direction. In a cross section a maximum width of the separation plug is located at a height Hb, which is less than ¾ of a height Ha of the separation plug.