A method of fabricating a CMOS thin film transistor includes: providing a substrate; forming an amorphous silicon layer on the substrate; performing a first annealing process on the substrate and crystallizing the amorphous silicon layer into a polysilicon layer; patterning the polysilicon layer to form first and second semiconductor layers; implanting first impurities into the first and second semiconductor layers; implanting second impurities into the first or second semiconductor layer; and performing a second annealing process on the semiconductor layers to remove the metal catalyst remaining in the first or second semiconductor layer, on which the second impurities are implanted, wherein the first impurities are implanted at a dose of 6×1013/cm2 to 5×1015/cm2, and the second impurities are implanted at a dose of 1×1011/cm2 to 3×1015/cm2.