The refractive index and absorption index of nanocrystalline diamond (NCD) films were investigated using spectroscopic ellipsometry between 30 and 500°C. Due to their high transparency the experimental spectra could be well fitted in the subgap region using a single-oscillator model with a four-phase layered structure. The single-oscillator model yields a small optical absorption in the band gap region. The temperature dependence of dispersion of the refractive index over the photon energy range of 1.15–4.75eV was determined. Based on the Bose-Einstein model, a thermo-optic coefficient of (1∕n)(∂n∕∂T)=6.5×10−6K−1 at 300K was obtained for the NCD film in the near-infrared region.