A blue-color-emitting phosphor, (Sr1-x,Ba-x)(3)MgSi2O8:Eu2+ (SBxMS:Eu2+), with various Ba contents, was synthesized, and its thermal stability was evaluated. Depending upon the Ba content, the dominant emission wavelength of the SBxMS:Eu2+ phosphor could be tuned, and good photoluminescence (PL) properties were obtained under an excitation by a 147 nm source. The PL behavior showed that the thermal stability of SBxMS:Eu2+ baked at 500 C was dependent upon the Ba content. On the basis of the results of electron spin resonance on Eu2+ and Rietveld refinement against neutron powder diffraction data of the phosphor, it was found that the improved thermal stability of SBxMS:Eu2+ could be ascribed to Eu2+ occupying preferred Sr sites among three possible locations depending upon the amount of Ba substitution. It is also inferred from this observation that the average interatomic distances between Eu2+ and O (d(Eu-o)) of the different Sr sites may play an important role in the thermal stability of the phosphor.