Lu2O3 films have been grown on Si(100) by atomic layer deposition using Lu(iPrO)3 (iPrO=OCH(CH3)2) and H2O. Optical properties and surface/interface evolution of the Lu2O3/Si system during rapid thermal annealing process have been studied using spectroscopic ellipsometry. The refractive index at 632.8 nm and optical band gap for as-deposited sample are determined to be 1.88 and 4.88 eV, respectively. It is revealed that interfacial Lu-silicate growth upon annealing is the dominant factor affecting the refractive index and optical band gap of Lu2O3 films. For all samples, the surface roughness decreases with increasing annealing temperature.