Si₂N₂O powder was synthesized by the direct reaction of silicon and silicon dioxide in an N₂ atmosphere at 1450°C; the resulting Si₂N₂O powder was doped with Eu in a H₂/N₂ atmosphere at 1350°C. The Eu-doped SrSi₂O₂N₂ powder showed a phosphorescence peak at approximately 530 nm with an excitation wavelength of 450 nm. Increasing the Eu content in the Eu_xSr_1−xSi₂O₂N₂ (x = 0–0.15) phosphor red-shifted the peak position. The Eu_0.01Sr_0.99Si₂O₂N₂ phosphor showed high durability in water, and the emission intensity of the Eu_0.01Sr_0.99Si₂O₂N₂ phosphor after soaking in water was 1.2 times higher than that of the phosphor before soaking in water.