Nitridoaluminosilicate MAlSiN3(M: alkaline-earth element) and its derivatives have attracted more and more attention owing to the fact that the material doped with rare-earth element has intense body color and exhibit efficient luminescence under InGaN diode irradiation. In particular, red phosphor, Eu-doped CaAlSiN3 (CASN), has good thermal property of luminescence and sufficient chemical durability for white LED use. Still, for the lineup of various kinds of white color, it is hoped to tune the red luminescence with other physical/chemical properties kept as possible. Thus the derivatives with different chemical compositions have been intensively explored so far. For the feasibility of such chemical composition change, it is necessary to understand its atomic/electronic structure of the unique crystal, which is a distorted AlN-based wurtzite superstructure (Cmc21, No.36) with Al and Si disordered on 8b site and Ca occupying 4a site. Recently, we have performed first-principles band calculation of CASN and clarified the origin of the Al/Si disorder configuration as well as the feasibility of the virtual crystal approximation of heterovalent cations (Al/Si) for the reproducibility of atomic/electronic structure of CASN. As a natural extension of this study, we have investigated some CASN-derivatives to confirm/predict the crystal structure. The VCA allows us to model the superstructure with various chemical compositions quite easily. In this work, we will present two examples of solid-solution, (Ca,Sr)AlSiN3 and CaAlSiN3-Si2N2O. The agreement between experiment and theory appears quite satisfactory. It is emphasized that the crystal structure of SrAlSiN3 has been successfully predicted by first-principles calculation prior to experimental result. The collaboration of experiment and theory promises us ‘gcrystal-engineering’ to develop new nitrides/oxynitrides effectively and efficiently.