Disclosed are a thin film transistor and a method of forming the thin film transistor, wherein the thin film transistor includes a gate electrode, an oxide semiconductor pattern, a first gate insulating layer pattern interposed between the gate electrode and the oxide semiconductor pattern, wherein the first gate insulating layer pattern has an island shape or has two portions of different thicknesses from each other, a source electrode and a drain electrode electrically connected to the oxide semiconductor pattern, wherein the source electrode and the drain electrode are separated from each other, and a first insulating layer pattern placed between the source electrode and drain electrode and the oxide semiconductor pattern, wherein the first insulating layer pattern partially contacts the source electrode and drain electrode and the first gate insulating layer pattern, and wherein the first insulating layer is enclosed by an outer portion.