Copper decoration followed by transmission electron microscopy (TEM) observation is proposed for identifying defects in thermal oxides of polysilicon films. This method uses copper decoration to detect the location of defects in the oxide, and sample thinning for TEM observation. The advantage of copper decoration is that the electronic current needed for defect detection is very small, so it does not change the original oxide-defect structure. TEM and scanning electron microscope observations revealed that the defects are polygonal voids located on the polysilicon grain boundary. The diameter of a void was typically 70 nm. A stress-induced migration of silicon atoms along grain boundaries may be related to the formation of the voids. A model for defect formation is presented.