SrAl₂O₄ : (Eu²⁺, Dy³⁺) phosphor was prepared by solid state reaction. B₂O₅ as a flux was added in SrAl₂O₄ : (Eu²⁺, Dy³⁺) in order to accelerate a solid state reaction. In this paper, the effects of B₂O³ on the crystal structure and the phosphorescent properties of the material have been evaluated. The synthesized phosphor exhibited a broad band emission spectrum peaking at 520 nm, and the spectrum peak showed little effect by the B₂O³ contents. The maximum afterglow intensity of the SrAl₂O₄ : (Eu²⁺, Dy³⁺) phosphor was obtained at the B₂O₃ content of 5%. Adding the B₂O₃ caused uniform distortion to the crystal structure of the phosphor and resulted in reducing the lengths of a and c axes and β angle of the SrAl₂O₄ crystal. The uniform distortion was accompanied with crystal defects which can trap the holes generated by the excitation of Eu²⁺ ions. The afterglow characteristic of the SrAl₂O₄ : (Eu²⁺, Dy³⁺) phosphor was thus enhanced.