The effect of strain and intrinsic defects on both electronic and structural characteristics of HfO2-x used for sub-100-nm semiconductor devices was analyzed by a quantum chemical molecular dynamics analysis. The magnitude of the band gap of HfO2 decreases by about 10% under the applied strain of 5%. The stable crystallographic structure of the monoclinic HfO2 changes to a cubic-like structure under the strain. The magnitude of the band gap of the HfO2-x decreases drastically from 5.7 eV to about 1.0 eV due to the formation of an electronic state within the band gap when an oxygen vacancy is introduced to the perfect HfO 2. In the HfO2-x film, oxygen atoms near the oxygen vacancy can move drastically at temperatures higher than 800 K. Therefore, it is very important to control the chemical composition of the hafnium oxide film and to optimize the annealing condition to maintain both the high reliability and performance of the gate oxide film.