Abstarct: In this research we synthesized and studied the surface morphology, structural, electrical and optical properties of molybdenum oxide MoO3 thin films prepared by spray pyrolysis method on glass substrate. For physical characterization of the samples we used field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), optical spectroscopy (UV-Vis.) and current-voltage (I-V) measurement. In the first step molybdenum oxide thin films were deposited with different spray rates of 1.5, 2.5 and 5 ml/min. It is found that compared to other samples, the one with the deposition rate of 5 ml/min has a better crystallinity. In order to improve this characteristic we investigated the effect of other deposition conditions including: modification in the spray route (section 4-2-1), concentration of the precursor (section 4- 3), and finally the substrate temperature (Section 4-4). In this study, we found that changing in substrate temperature lead to the optimum condition, from crystallinity point of view, for the MoO3 laxyer.