The present application provides a method of fabricating a thin film transistor. The method includes selecting a nano-structure material having a monotonic relationship between a threshold voltage and a channel length when the nano-structure material is formed as a channel part in a thin film transistor; forming an active layer using the nano-structure material; determining a nominal channel length of a channel part of the thin film transistor based on the monotonic relationship and a reference threshold voltage so that the thin film transistor is formed to have a nominal threshold voltage; and forming a source electrode and a drain electrode thereby forming the channel part in the active layer having the nominal channel length.