Identifying a crystal structure of alumina (Al2O3) scale is critical for evaluating the performance of heat-resistant alloys because α-Al2O3 is stable and protective against high-temperature oxidation and corrosion but θ- and g-Al2O3 provide poor oxidation resistance. Conventional methods to identify crystal structures of Al2O3 scales are time-consuming. Herein, the author proposes a method to rapidly identify crystal structures of Al2O3 scales on β-NiAl by obtaining their cathodoluminescence (CL) spectra. α-, θ-, and g-Al2O3 can be identified by detecting a sharp peak at 695.8 nm and 686.3 nm, and a broad peak at around 700 nm, respectively, in CL spectra. Concentrations of α-, θ-, and g-Al2O3 scales can be determined roughly from intensities of these peaks. This method can be applied to areas ranging from the millimeter to micrometer scale, and the acquisition time for the CL spectra was less than 10 s. The results indicate that obtaining CL spectra contributes to the identification of crystal structures of Al2O3 scales on heat-resistant alloys and a reduction in time to evaluate the performance of heat-resistant alloys.