We have calculated the atomistic mechanism for the HfO2 atomic layer deposition (ALD) using Hf(NEtMe)(4) and H2O precursors using density functional theory. On hydroxylated Si surface, our results show overall Hf(NEtMe)(4) half-reaction is exothermic by 1.65 eV with a small activation barrier of 0.10 eV. The activation barriers for water half-reaction are 0.24 and 0.20 eV. This indicates HfO2 ALD with Hf(NEtMe)(4) and H2O has a faster deposition rate than that with HfCl4 and H2O and can be run at relatively low temperature. However, on H-terminated Si surface, the Hf(NEtMe)(4) half-reaction is endothermic by 2.39 eV with high activation barriers. The H2O half-reaction is similar to that on hydroxylated surface which is kinetically favorable. The results suggest that long Hf(NEtMe)(4) pulse time and high deposition temperature is required during the initial stage of ALD on H-terminated surface. Moreover, our calculations indicate the reaction byproduct HNEtMe is likely to be bound to the surface after each half-reaction because of high desorption energy of 0.7-0.9 eV. Thus, long precursor purge time should be used if HNEtMe is needed to be removed completely.