We investigated the evolution of the physical and electrical properties of HfO2 thin films deposited by the reactive DC magnetron sputtering method on the (100) silicon substrate upon annealing. The HfO2 thin films deposited at room temperature were amorphous, while the films after annealing were poly crystalline. The crystallization temperature of the HfO2 thin films was dependent on the annealing methods (RTP or Furnace) and ambient (nitrogen or oxygen). The microstructures of HfO2 thin gate oxides of Tox <15A thick were analyzed by HRTEM, XPS and XRD. We also focused on the interfacial layer between HfO2 thin films and silicon substrates. Due to its high oxygen diffusivity, any annealing led to oxygen diffusion rapidly through the HfO2 films. The EOT increased upon annealing due to the increased SiO2-containing layer. The HfO2 thin films deposited at room temperature have the undesirable interfacial states due to the surface damage by sputtering. We found that the HfO2 thin films <15A thick were optimized by RTP or furnace annealing.