In this paper, we have proposed a new method for the study of disturb failure mechanisms caused by stress induced leakage current (SILC) in source-side erased flash memories. This method is able to directly separate the individual components of SILC due to either carrier charging/discharging in the oxide or the positive charge/trap assisted electron tunneling into the floating gate. In addition, the present method is very sensitive with capability of measuring ultralow current (<10-19 A). Results show that, at low oxide field, the disturb is mainly contributed by the so-called charging/discharging of carriers into/from the oxide due to the capacitance coupling effect. While at high oxide field, the positive charge/trap assisted electron tunneling induced floating-gate charge variation is the major cause of disturb failure.