Rotating bending fatigue tests were carried out using a radical nitrided nickel base superalloy, Alloy 718, at 500°C, to investigate the effect of radical nitriding on the initiation of a fatigue crack at elevated temperature. The nitriding was conducted after normal double-aging at two different conditions, i.e. at 500°C for 12h and at 570°C for 20h, in mixed gases of hydrogen and ammonia. By the nitriding, compound layers were formed on the specimen surface with the thickness of about 4μm at 500°C and 10μm at 570°C, respectively. Fatigue strength at 500°C was larger than that at room temperature in both nitrided alloys in wide life region. Fracture origins of the nitrided alloys changed from the base alloy beneath the compound layer to the interior of the alloy with decreasing in stress level, causing that S-N curves showed a step wise shape similar to the result of the aged alloy. The crack initiation was suppressed by the compound layer. Fatigue strength was increased by nitriding in the horizontal region in S-N curves where fatigue strength was controlled by the crack initiation and the propagation of a small crack, though the effect of nitriding on fatigue strength was very small in the region where fatigue life was occupied by the growth of a crack, i.e. in the region except for the horizontal one in S-N curves. Although a surface crack in the nitrided alloy propagated in a brittle manner, the crack propagated steadily similar to the one in the aged alloy propagated in a ductile manner.