Molybdenum oxide (MoO3) thin films have been obtained by plasma-enhanced atomic layer deposition using precursors of Mo(CO)6 and O2 plasma at a reactor temperature of 160 °C. The MoO3 thin films deposited on p-type silicon substrates with thermally grown 300 nm thick SiO2 layer with different numbers of ALD growth cycles. Spectroscopic ellipsometer is used to characterize as-grown film thickness and refractive index. The as-grown films are treated at annealing temperatures at 300–600 °C. The structural properties of as-grown and annealed thin films have been characterized by optical microscopy, µRaman spectra, and atomic force microscopy.