In order to clarify the influence of water vapor on evaporation and crystallization of an amorphous SiO₂ scale formed on SiO₂-formers, fused silica was exposed for up to 49 h at 1373—1673 K in N₂-O₂-H₂O atmospheres. The water vapor concentration was regulated to 19.6 and 46.7% (vol%). Under conditions of low water vapor concentration, the mass change in the fused silica was negligible. On the other hand, the mass linearly decreased with the duration of exposure under high water vapor concentration. From the temperature dependence of the evaporation rate, it was speculated that the main volatile species were Si(OH)₄ and SiO(OH)₂ in N₂-H₂O and N₂-O₂-H₂O, respectively. Moreover, although crystallization of fused silica was observed in both atmospheres, air and N₂-O₂-H₂O, the rate of crystallization in the N₂-O₂-H₂O atmosphere was much higher than that in air. These results indicate that crystallization is accelerated by the presence of water vapor.