Abstract The optical properties of blue emitting BaAl 2S 4:Eu thin-films were studied. The emission peak is around 470 nm, whose FWHM (full width at half maximum) is 35 nm. The dielectric constant is 3.03 from transmission spectrum, and the optical band gap is approximately 4.6 eV. It is found that the cubic phase and orthorhombic phase exist in fabricated thin-films. Furthermore, the energy band structure of BaAl 2S 4:Eu thin-films from X-ray photo-electron spectra (XPS) and the absorption spectra were analyzed.