Thin NiO films are grown at 300 °C on Si (100) using atomic layer deposition. The dependence of annealing temperature on the optical properties of NiO films has been investigated using spectroscopic ellipsometry in the spectral region of 1.24–5.05 eV. It is found that the refractive index and thickness of NiO films are affected by high temperature annealing. The optical band gap of the as-deposited thin NiO film is determined to be 3.8 eV, which is almost independent of the annealing temperature. The indirect band gap of NiO film shifts toward lower photon energy with an increase in annealing temperature.