β-(AlxGa1-x)2O3 presents great applications in modern power devices and deep ultraviolet photoelectric detection for their excellent anti-breakdown and tunable band gap. However, the complexity and difficulty of the traditional fabrication processes limit their further development. In this work, a relatively simple high temperature diffusion process was used to successfully prepare β-(AlxGa1-x)2O3 nano films on c-sapphire substrates. The films were investigated by X-ray diffraction, atomic force microscope, scanning electron microscope, and ultraviolet visible spectrophotometer. Since Al atoms in sapphire substrates will diffuse into the Ga2O3 layer at high temperatures, β-Ga2O3 thin films will be converted into β-(AlxGa1-x)2O3 thin films with different ratios of Al to Ga atoms. It illustrates that with the increase of annealing temperature from 1 010 ℃ to 1 250 ℃, the average content of Al in the films increases from 0.033 to 0.371. Meanwhile, the thickness of films increase from 186 nm to 297 nm, accompanied by the roughness increase from 2.31 nm to 15.10 nm with the increase of the annealing temperature from 950 ℃ to 1 250 ℃. While increasing the annealing temperature from 950 ℃ to 1 190 ℃, the band gap of films increases from 4.79 eV to 5.96 eV. The results suggest that the high temperature diffusion process can effectively adjust the optical band gap of β-(AlxGa1-x)2O3 thin films, providing an experimental basis for novel β-(AlxGa1-x)2O3-based optoelectronic devices.