A process is disclosed for preparing M' YTaO.sub.4 :Nb x-ray phosphor which comprises forming a uniform first mixture of Y.sub.2 O.sub.3, Ta.sub.2 O.sub.5, and Nb.sub.2 O.sub.5 in amounts equal to approximately the stoichiometric amounts to form the phosphor, milling with a flux of lithium chloride, the flux making up about 25% to 33% by weight of the mixture, firing the mixture in a furnace by heating to a temperature of about 1200.degree. C. to 1300.degree. C. at a heating rate of about 1.0.degree. C./minute to 1.5.degree. C./minute and maintaining the temperature for about 10 to 14 hours to react the components and produce a fired material containing luminescent material, cooling the material by turning off the heat and allowing the material to remain in the furnace until the temperature is no higher than about 300.degree. C., washing with deionized water, removing the wash water, drying, and classifying to obtain a -325 mesh particle size. The phosphor exhibits improved brightness over that of phosphors produced by methods using water or organic solvents as the milling fluid and substantially reduces crucible corrosion. Reduced amount of Nb.sub.2 O.sub.5 activator concentration has a beneficial effect on phosphor brightness.