The Al doped Sr2Si5N8:Eu2+ single crystal Sr2AlxSi5-xOxN8-x:Eu2+ (x approximate to 0.7) were grown by our original single crystal growth method and the precise crystal structure was investigated by single crystal X-ray diffraction analysis for the first time. O atoms are partially substituted for only N1 and N5 sites which are connected with the Si/Al atoms to compensate the "Pauling's second rule", which indicates the introduction of Al atoms for Si sites restricts the N sites that can be substituted by O atoms. The Sr2AlxSi5-xOxN8-x:Eu2+ (x approximate to 0.7) phosphors show the change of site selectivity of Eu for Sr sites, which causes the red-shifted emission spectrum of Sr2AlxSi5-xOxN8-x:Eu2+ (x approximate to 0.7) than that of Sr2Si5N8:Eu2+. (C) 2019 The Japan Society of Applied Physics