For the first time, a difference analysis method has been applied to separate and characterize interface and oxide traps generated in an ultra-thin direct tunneling (DT) gate oxide. It provides a useful tool to study the mechanism of degradation in ultra-thin MOSFET and extract parameters of interface trap and oxide trap, such as the generation/capture cross-section, centroid and density when a large DT is injected through the gate oxide. This method is an extension of oxide trap relaxation spectroscopy (OTRS) technique. It has the advantage of being direct, fast and convenient in the study of MOSFET reliability.