The relationship between electrical and structuralcharacteristics of polycrystalline HfO2films has been investigated by conductive atomic force microscopy under ultrahigh vacuum conditions. The results demonstrate that highly conductive and breakdown (BD) sites are concentrated mainly at the grain boundaries (GBs). Higher conductivity at the GBs is found to be related to their intrinsic electrical properties, while the positions of the electrical stress-induced BD sites correlate to the local thinning of the dielectric. The results indicate that variations in the local characteristics of the high-k film caused by its crystallization may have a strong impact on the electrical characteristics of high-k dielectric stacks.