A sensitive and simple method to detect process-induced initial defects in fresh ultra-thin gate dielectrics is proposed, where only variations of gate leakage currents are estimated at a low gate voltage. This method proves to be a powerful tool for choosing the suitable manufacturing conditions to suppress the initial defects and predicting reliabilities of MOSFETs. The measurement conditions were determined from detailed studies of characteristic behaviors of the leakage currents through the initial defects in the wide voltage region. The conduction behaviors of gradually increasing gate currents during a progressive breakdown event are also investigated to attempt to extend the application range of this method. The currents through the breakdown spots also show characteristic conduction behaviors similar to the currents through the initial defects.