Abstarct: In this thesis the surface morphological, structural and optical properties of molybdenum trioxide (MoO3) thin films undoped and doped with Zn (at different atomic percentages of 1, 5, 10, 20%), prepared using spray pyrolysis and hydrothermal methods, were studied. For the characterization of prepared samples field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray fluorescence and UV-Vis spectroscopy equipment were utilized. The XRD pattern analysis shows the thermodynamically stable orthorhombic α-MoO3 phase for undoped samples prepared by spray pyrolysis. Strong reflection peaks was observed at (0k0) with k=2, 4, 6 in XRD patterns, which proves the lamellar structure of molybdenum trioxide in the prepared thin films. The FESEM images of samples show the formation of micro-spheres which their shell was shaped in lamellar structure. By increasing the amount of dopant, changes in crystal structure from orthorhombic to monoclinic was observed, however there are complex phases. Also, the transmittance, absorbance coefficient, and direct band gap of samples were obtained and analyzed, using transmission and absorption spectra of films. Undoped thin films which were synthesized using spray pyrolysis, were utilized as seed laxyers in hydrothermal technique for growing nanostructured thin films undoped and doped with Zn (at different atomic percentages of 2, 5, 10%). In this case, XRD pattern analysis revealed the formation of mexta-stable hexagonal h-MoO3 crystal structure, and the FESEM images illustrated the preferential grows of well-arranged hezagonal micro- and nano-rods on top of sprayed seed laxyers. By increasing in the amount of dopant, the diameter of rods increased and their length decreased. Also optical properties of samples investigated. At the end, the sensing properties of samples prepared by spray pyrolysis (at different dopant concentrations) was measured towards different concentrations of ethanol vapor. Measurements was performed at the temperature range of of 200 - 300 °C. The highest response belongs to the sample doped with 5 at.% Zn measured at 250 °C, which is 23% for 100 ppm of ethanol vapor.