The effects of the sodium-water reaction by small water/steam leaks on 2.25Cr-1Mo and Mod.9Cr-1Mo steels have been studied. Test specimens were exposed to small leaks of water/steam in 300 ℃ stagnant sodium. The phenomenon where the size of the nozzle hole became larger with time was observed for the two types of material. Enlargement rate was slightly faster in the 2.25Cr-1Mo steel than in the Mod.9Cr-1Mo steel. Test results of the enlargement rate of the nozzle hole itself with an increasing duration of the steam injection were analyzed by using SEM and CAMSCOPE images. The maximum reaction temperature appeared at a 17 mm point from the measuring tool, which agrees with the maximum distance that was affected by a jet flame.