The conduction mechanism of ultra-thin gate oxide n-metal-oxide-semiconductor fi eld effect transistor (n-MOSFET) after soft breakdown is studied in this paper. It is found that in a certain range of gate voltage Vg, the gate cur r ent Ig follows the Fowler-Nordheim-like tunneling mechanism, and the experimen tal tunneling barrier b is 0936 eV in average, which is much smal ler t han the interface barrier of Si/SiO2 We think that after soft brea kdown, the electrons existing in the quantization energy levels of the Si/SiO2 interface , not directly tunnel to the oxide conduction band, but tunnel to the oxide defe ct band. b is determined by both the defect band energy level and t he qu antization energy level of the tunneling electrons. With rising experimental tem perature, the tunneling of high energy level electron is also increasing, which reduces b gradually.