Rotating bending fatigue tests up to 108 cycles were carried out to investigate the effects of shot peening on the fatigue strength and the fracture mechanism in an 18 % Ni maraging steel by using shot particles of various sizes or hardness. Fatigue strength was increased markedly by shot peening in the wide region of fatigue life. The S-N curves showed duplex S-N properties because of the transition of fracture origin from the specimen surface in the short life region to the subsurface in the long life one. Double shot peening by using super-hard fine particles was effective to improve the fatigue strength for surface fracture, though the fatigue strength for an internal fracture was hardly influenced. These results were discussed from the points of view of effects of surface roughness, residual stress and work hardening on the fatigue strength.