In recent years, MoSe ₂, as a kind of transition metal dichalcogenides have been attracting a wide range of research interests due to its special crystal structure which exhibits different electrical and optical properties. The band gap of molybdenum diselenide can be manipulated by different layers, strain engineering, doping, or the formation of heterostructures, which makes it potentially advantageous in optoelectronic devices and photovoltaic applications. In this work, we investigate the effect of selenization temperature on the structure and optical properties of the MoSe ₂films. Molybdenum (Mo) thin films were prepared by RF magnetron sputtering, and then MoSe ₂thin films were generated by selenization annealing. The surface morphology, crystal structure, and optical bandgap of the MoSe ₂thin films were characterized and analyzed using scanning electron microscopy, X-ray diffraction, and ultraviolet visible spectroscopy, respectively. The results show that the crystal structure of the MoSe ₂thin films is closely related to the selenization temperature (T _s): with the increase of selenization temperature, the average grain size of the thin films decreases slightly and then increases rapidly (from 24.82 nm to 55.76 nm). Meanwhile, the (002) crystal plane of MoSe ₂also exhibits preferential growth with increasing temperature. The MoSe ₂thin films have a low absorption rate for short-wavelength light (around 600 nm). With the increase of selenization temperature, the bandgap wave of the MoSe ₂thin films is blue-shifted, and the optical bandgap decreases. The reason is that different selenization temperatures cause changes in the lattice size of MoSe ₂, thereby affecting the spatial expansion of its electronic wave function. In addition, the structure and optical bandgap of MoSe ₂can be effectively controlled by changing the selenization temperature, which provides more possibilities for the MoSe ₂thin films in the application of optical devices.