In this study, iron-doped tungsten oxide (Fe: WO₃) thin films were synthesized on p-type silicon (Si (111)) and glass substrates using a DC/RF co-magnetron sputtering technique under controlled deposition conditions. The effect of post-deposition annealing on the structural and crystallographic evolution of the films was systematically examined at temperatures of 550°C, 650°C, and 750°C in air. X-ray diffraction (XRD) analysis revealed that the as-deposited films were poorly crystalline. In contrast, progressive annealing resulted in a significant enhancement in crystallinity, accompanied by the formation of a well-defined monoclinic WO₃ phase. Increasing the annealing temperature led to sharper diffraction peaks and larger grain sizes, confirming thermally induced grain growth and improved structural ordering. A noticeable shift of diffraction peaks toward higher 2θ values was observed with Fe incorporation, indicating lattice contraction due to the substitution of W⁶⁺ ions by smaller Fe³⁺ ions, as well as the generation of oxygen vacancies for charge compensation. The average crystallite size increased from 21.6 nm at 550°C to 62.4 nm at 750°C, demonstrating that Fe doping promotes thermally assisted grain coalescence. These results suggest that Fe incorporation effectively alters the lattice parameters and structural stability of WO₃ without creating any secondary Fe-based oxide phases. Overall, these findings provide valuable insights into the relationship between microstructural evolution and dopant-induced modifications in Fe: WO₃ systems, paving the way for optimizing their performance in photocatalytic and gas-sensing applications.