The decay of the luminescence from Cr3+ ions in mixed-cation (Na1-xAgx and Na1-xLix) beta-alumina crystals was measured to study the disorder characteristics of the conduction plane. The decay curves of the luminescence bands obtained for all samples cannot be represented by using a combination of the exponential functions, but require that of the extended-exponential function. This result will be related to a disordered character of the conduction plane. Moreover, from the analyses of the decay feature and the electron-phonon interaction, it is suggested that the two kinds of conduction ions in the conduction plane for samples near x = 0.4 of Na1-xAgx beta-alumina, in which the conductivity is remarkably lower, make a special structure.