The mechanism of luminance decrease in blue emitting BaMgAl10O17:Eu2+ phosphor after annealing has been studied. It was shown experimentally that annealing in air oxidizes BaMgAl10O17:Eu2+ and substantially changes the Eu2+ ion to an Eu3+ ion. This is also deactivates the Eu2+ luminescent center, thus decreasing Eu2+ emission. Furthermore, the compound EuMgAl11O19, in which the valence of the Eu ion is trivalent, was detected in BaMgAl10O17:Eu2+ annealed at temperatures higher than 800 degrees C as a resulting product of oxidation. It was demonstrated that BaMgAl10O17:Eu2+ was converted by oxidation into a mixture of the two compounds BaMgAl10O17 and EuMgAl11O19, and resulting in a further decrease in luminance. This suggests that obstructing the formation of Eu(III)MgAl11O19 during the annealing is essential for alleviating the decrease in luminance. Preparing BaMgAl10O17:Eu2+ having both superior crystallinity and completely stoichiometric composition has been found to be effective in alleviating the luminance decrease.