A process is disclosed for producing M' phase YTaO.sub.4 :Nb X-ray phosphor which comprises forming a mixture of the components Y.sub.2 O.sub.3, Ta.sub.2 O.sub.5, and Nb.sub.2 O.sub.5, and a flux which can be lithium chloride, lithium sulfate or a mix of lithium sulfate and potassium sulfate in a mole ratio of 80 to 20, the flux making up about 25% to 50% by weight of the mixture, the components being provided in an amount equal to approximately the stoichiometric amounts to form the phosphor, 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./min. to 1.5.degree. C./min. 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 to the furnace 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 and drying, and classifying to obtain a -325 mesh particle size in the phosphor, forming a mixture of the phosphor and flux wherein the flux makes up about 10 to 50% by weight of the mixture, firing as before with the temperature being about 1100.degree. C. to 1300.degree. C. and the time being about 2 to 10 hours, cooling, washing, drying and classifying to produce a phosphor having a higher luminescence intensity than phosphors produced absent the second firing step.