Effect of Laves phase precipitation on the fatigue behavior in JFE 429EX ferritic stainless steel was investigated. When this material is exposed to the temperature range of 700-900°C, the precipitation of Laves phase (Fe₂Nb) occurs, resulting in the embrittlement of material known as Laves phase embrittlement. Axial loading fatigue tests have been conducted in laboratory air and in 3%NaCl solution using a niobium-containing ferritic stainless steel, JFE 429EX, aged at 700, 800 or 900°C. In laboratory air, the fatigue strengths of 700°C-aged specimen were improved by the fine Laves phase precipitation into α-phase. Whereas the fatigue strengths of 800 and 900°C-aged specimens decreased, which was attributed to the coarsening of Laves phase precipitates. On the other hand, in 3%NaCl solution, the fatigue strengths of all aged specimens were lower than those in laboratory air. That was because sensitization took place during aging, that is, the chromium-depleted zones were formed along grain boundaries.