Recently, InxAlyGa1−x−yN quaternary alloys have attracted much research interest because of the great flexibility in tailoring their band gap profile while maintaining their lattice-matching and structural integrity. In this paper, we present the study of crystalline quality of InxAlyGa1−x−yN quaternary alloys by using photoluminescence (PL) spectroscopy and X-ray diffraction (XRD) technique. The InxAlyGa1−x−yN quaternary alloys were grown on sapphire by molecular beam epitaxy with indium (In) mole fraction, x, ranging from 0 to 0.1 and constant aluminium (Al) mole fraction (y = 0.06). Through this study, the variation of crystalline quality of the InAlGaN epilayers as a function of In composition is investigated by analyzing the full width at half maximum (FWHM) of the PL peak and (0002) InxAlyGa1−x−yN diffraction peaks. The results revealed that the FWHM of the PL and the (0002) InxAlyGa1−x−yN diffraction peaks gradually increased as the In composition is increased indicating the decreased of the sample's quality. The broadening may probably be attributed to the point defects leading to non-uniformity of the quaternary layers.