A semiconductor device including a device isolation region is provided. The semiconductor device includes first active regions disposed on a substrate, and an isolation region between the active regions. The isolation region includes a first portion formed of a first insulating material, and a second portion formed of a second insulating material, having different characteristics from those of the first insulating material. The first portion is closer to the first active regions than the second portion. The second portion has a bottom surface having a height different from that of a bottom surface of the first portion.