A thin film transistor array substrate of a thin film transistor liquid crystal display (TFT-LCD) is provided. The gate dielectric layer of the TFT includes a silicon nitride layer, a dielectric layer and a silicon nitride layer, and the etching selectivity of the amorphous silicon layer over the dielectric layer is not less than about 5.0. Therefore, the dielectric layer can be an etching stop layer when doped and undoped amorphous silicon layers are etched to form source/drain stacked layers or a conductive layer is etched to form a gate on the gate dielectric layer. Hence, the dielectric layer thickness can be controlled, and thereby the capacitance of the storage capacitor can be controlled.