A thin film transistor array substrate having a high charge mobility and that can raise a threshold voltage, and a method of fabricating the thin film transistor array substrate are provided. The thin film transistor array substrate includes: an insulating substrate; a gate electrode formed on the insulating substrate; an oxide semiconductor layer comprising a lower oxide layer formed on the gate electrode and an upper oxide layer formed on the lower oxide layer, such that the oxygen concentration of the upper oxide layer is higher than the oxygen concentration of the lower oxide layer; and a source electrode and a drain electrode formed on the oxide semiconductor layer and separated from each other.