A field effect transistor includes a fin structure, having a sidewall, protruding from a substrate, and a device isolation structure on the substrate, the device isolation structure defining the sidewall of the fin structure, wherein the fin structure includes a buffer semiconductor pattern disposed on the substrate and a channel pattern disposed on the buffer semiconductor pattern, wherein the buffer semiconductor pattern has a lattice constant different from that of the channel pattern, and wherein the device isolation structure includes a gap-fill insulating layer, and includes an oxidation blocking layer pattern disposed between the buffer semiconductor pattern and the gap-fill insulating layer.