Molybdenum thin films with 50 and 150 nm thicknesses were deposited on silicon substrates, using DC magnetron sputtering system, then post-annealed at different temperatures (200, 325, 450, 575 and 700°C) with flow oxygen at 200 sccm (standard Cubic centimeter per minute). The crystallographic structure of the films was obtained by means of x-ray diffraction (XRD) analysis. An atomic force microscope (AFM) and a scanning electron microscope (SEM) were employed to investigate the samples surface morphology. A four point probe instrument and a spectrophotometer were used for electrical resistivity and transmittance spectrum measurements, respectively. XRD results showed a MoO3 polycrystal (single phase) for annealed samples at 450 and 575°C, while annealed sample at the highest temperature (700°C) had a mixed phase of MoO3 and Mo9O26. Both the grain size and the surface roughness of the samples increased with annealing temperature. The electrical resistivity and transmittance spectrum of the films increased with increasing of annealing temperature up to 575°C, while an increase in annealing temperature to 700°C had an inverse effect. It was also observed that the thicker films (with 150 nm thickness) had lower resistivity and transmission.