A method of crystallizing an amorphous semiconductor thin film formed on a substrate is provided. The method includes the steps of: forming a gate insulation film and a gate electrode on an amorphous semiconductor thin film; locally forming first and second crystallization induced metal patterns for inducing crystallization of the amorphous semiconductor thin film, on part of the amorphous semiconductor thin film spaced at a predetermined off-set distance from the gate insulation film; ion-injecting impurities into the substrate to thus define a source/drain region; forming a protection film on the whole surface of the substrate; and heat-treating the substrate in the air to thereby crystallize the amorphous semiconductor thin film. As a result, the protection film such as an oxide film is coated in advance before a metal induced lateral crystallization (MILC) heat treatment when the amorphous semiconductor thin film is crystallized, to thereby enabling the MILC heat treatment even in the air as well as under the inert gas, hydrogen, or vacuum atmosphere, and to thus save a cost for maintaining a heat treatment atmosphere.