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Low cycle fatigue tests were performed on IN 718-DA alloy at 600°C, both inair and under vacuum environment. The effect of an hold time (90s) at maximum appliedtotal strain was investigated. All the specimens were taken from forged discs. These testresults were compared to those obtained on conventional 718 (annealed and aged) testedunder vacuum. SEM observations of the fracture surfaces and of the gauge length of thespecimens showed various types of initiation sites. Both in air and under vacuum, thefatigue cracks initiated from surface connected particles (carbides and nitrides) or fromsubsurface particles. The deleterious effect of oxidizing environment on the fatigue life isexplained by a preferential oxidation of carbide particles. A mechanism accounting forcleavage of nitride particles due to preferential carbide oxidation is proposed.