High-luminescent (Y,Gd)BO₃:Eu red phosphor particles were synthesized by spray pyrolysis and the morphology of these particles was improved by controlling the precursor solution to be sprayed. An investigation was also conducted into the effects that several codopants (In, Dy, Sm, Tm, and Pr) have on the photoluminescence intensity of (Y,Gd)BO₃:Eu particles under vacuum ultraviolet excitation. It was found that modifying the spray solution with NH₄OH improved the morphology of phosphor particles obtained after heat treatment. As a result, the rod-shaped (Y,Gd)BO₃:Eu particles obtained from the nitrate precursor solution in the spray pyrolysis were almost spherical. By varying the heat treatment temperature from 900 to 1200℃, the optimal temperature was found to be 1050℃ in terms of the luminescence efficiency and morphology of the (Y,Gd)BO₃:Eu particles prepared from the NH₄OH-modified precursor solution. Of the several codopants used, indium was the most effective at improving the photoluminescence intensity of (Y,Gd)_(0.95)BO₃:Eu_(0.05),M_(x) particles under vacuum ultraviolet excitation. Finally, the highest photoluminescence intensity was obtained when the indium content (X) was 0.001. The optimized (Y,Gd)_(95)BO₃:Eu_(0.05),In_(0.001) particles show a 6% higher luminescence intensity than do the commercial particles.