[ $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ ] phosphor particles with high photoluminescence intensity under long wavelength ultraviolet were prepared by spray pyrolysis. We investigated the effect of $NH_4F$ flux added into starting solution on the morphology and photoluminescence intensity of $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ phosphor prepared by spray pyrolysis. $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ phosphor particles prepared from starting solution containing $NH_4F$ flux had the maximum photoluminescence intensity at the post-treatment temperature of $1200^{circ}C$ and the maximum photoluminescence intensity of $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ phosphor particles prepared from the starting solution containing $NH_4F$ flux was $137\%$ of that of the phosphor particles prepared from the starting solution without flux material. $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ phosphor particles prepared from starting solution containing $NH_4F$ flux had larger size and more aggregated morphology than those prepared from starting solution without flux material. The photoluminescence intensity of $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ phosphor particles prepared from starting solution containing $NH_4F$ flux above $3wt.\%$ had high photoluminescence intensities. The addition amount of $NH_4F$ flux showing the maximum photoluminescence intensity was $12wt.\%$. The optimum amount of $NH_4F$ flux was $5wt.\%$ when we considered the morphological and photoluminescence characteristics of $Ba_{1.5}Sr_{0.5}SiO_4:Eu$ ohosphor particles prepared by spray pyrolysis.