Electric current induced breakdown phenomenon of silicon dioxide film was studied by annealing the sandwich samples of polycrystalline Si/SiO 2 /Si substrate at 700 °C under electric currents. Cross-sectional transmission electron microscopy revealed two interesting results: (1) the aggregation of voids into large holes at the interface of polycrystalline Si/SiO 2 , and (2) the nucleation and growth of crystalline Si in the silicon dioxide layer. The crystalline Si was either of single crystalline or polycrystalline forms, depending on the nucleating interfaces. Local melting as a result of Joule heating was observed. The breakdown of the oxide is mainly attributed to the loss of oxide integrity as a result of heterogeneous Joule heating. The nucleation and growth of crystalline Si in the amorphous oxide layer suggest the direct decomposition of SiO 2 during the oxide breakdown process.