Fine size BaMgAl10O17:Eu phosphor powders were prepared by spray pyrolysis from a spray solution with ethylenediaminetetraacetic acid, citric acid, and NH4F flux. The precursor powder prepared by spray pyrolysis has a hollow, thin-walled structure and was transformed into BaMgAl10O17:Eu phosphor powders with fine size and a plate-like morphology. The phosphor powders had pure hexagonal BAM phases even at a low temperature of 1100°C. The mean sizes of the phosphor powders prepared from a spray solution with 6wt. % NH4F of the phosphor were 1.1 μm, 2.4 μm, and 1.9 μm at post-treatment temperatures of 1100°C, 1150°C, and 1300°C. The maximum photoluminescence intensity of the phosphor powders post-treated at 1150°C was 93% of that of commercial BAM:Eu phosphor powders.