A simple model which links the primary hole and Fowler-Nordheim (FN) electron injections to oxide breakdown is established and the calculation based on this model is in good agreement with our experiments. When the sum of the active trap density D-pri due to primary hole injection and the active trap density D-n due to FN electron injection reaches a critical value D-cri, the oxide breaks down. The hole is two orders of magnitude more effective than FN electron in causing; breakdown. These new findings are imperative in predicting oxide reliability and device lifetime.