A method of fabricating poly crystalline silicon type thin film transistor is disclosed. In the method, before the step of re-crystallization of amorphous silicon to form polycrystalline silicon active pattern, a step for injecting predetermined amount of oxygen atom into the surface part of the amorphous silicon layer. By this addition of step, the surface part of the silicon layer is to be oxidized and the crystal defect in the interface between the gate insulating layer and poly crystalline silicon layer can be cured and the mobility of charge carrier can be improved in the channel of the thin film transistor.