<p>The work affected layer of shot peened maraging steel was examined and fatigue properties of the steel were investigated under rotating bending in relative humidity of 25 % and 85 %. Shot peening produced a grain refinement in the surface layer with about 50 μm in depth of a specimen and a phase transformation to austenite structure was measured on the specimen surface, in addition to hardening and formation of compressive residual stress. Especially, the thin surface layer showed a nano-size structure in the work affected layer. Fatigue strength and its humidity sensitivity were improved by shot peening. The fatigue fracture process was occupied by the initiation and propagation of a surface crack in the electro-polished specimens. On the other hand, fracture mode in the shot-peened specimens smoothly changed from the fracture controlled by the propagation of many surface cracks originate at dimples yielded by shot peening at high stress levels to the combined fracture accompanied the propagation of an internal crack originated from a subsurface defect of a specimen with the surface crack propagation simultaneously with decreasing in the stress level. A definite internal fracture controlled by a propagation of an internal crack was not confirmed until around 10⁷ cycles. In relation to the change in fracture mode stated above, fatigue life increased with decreasing in the stress level monotonically, that is, the shape of S-N curve of the shot-peened specimen did not show a duplex curve which was observed in many surface-treated steels. In all of the macroscopic fracture surfaces, a ring-shaped flat area was observed at the surface layer of the shot-peened specimens and the depth of the area nearly corresponded to the thickness of the fine-grained layer. No or little effect of humidity on the above-mentioned results was confirmed.</p>