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The shot peening process in normal conditions produces a compressive residual stress on the surface of a material without phase transformation. Instead, shot peening induces a change in phase (e.g. nanoferrite layer and metal flow layer) on the surface of carbon steel under the intensified peening conditions associated with higher peening velocities and the use of a high hardness shot media.This study investigated the effect of shot peening time on changes in surface microstructure and its effect on fatigue strength. The test specimens were compressive coil springs made from oil tempered wire. The test springs were manufactured by the same process as that used for the test specimens except for the shot peening condition. A total of seven shot peening times were employed: 100, 300, 500, 1000, 2000, 3000, 6000 s. The projected material was 0.25 mm diameter steel cut wire. The test springs were measured for surface roughness and residual stress distribution observed using an optical microscope and scanning electron microscope.Fatigue tests were conducted using a spring fatigue test machine at a frequency of 30 Hz for 2×10 7 cycles and a test stress of t m ±t a ＝600±540 MPa.Values of the residual stress distribution for 300 6000 s were similar, as was surface roughness of all the springs as peening was performed using the same media.The spring for the 300 s projection consisted of the matrix phase and the surface metal flow phase. Fatigue strength increased until a shot peening time of 1000 s, after which levels remained constant. The spring microstructure for the 1000 s projection revealed a white layer on part of the surface. Durations exceeding 3000 s were associated with the occurrence of the white layer, which covered entire the surface and was not associated with any increase in fatigue strength.