Influence of stress ratio on bending fatigue limit of high strength steel (SUP9A) containing a semi-elliptical surface slit subjected to shot peening (SP) was investigated. SP was conducted on the specimens containing a semi-circular surface slit with the crack depth (a=0.1, 0.2 and 0.3 mm). Bending fatigue tests were carried out under stress ratio, R = －1. These results showed that the fatigue limit of the shot-peened specimens having the slit under 0.2 mm in depth was almost same as that of the shot-peened smooth specimens. Meanwhile, some of the specimens fractured from the surface other than the slit. Thus, the maximum depth of slit which could be rendered harmless by SP was 0.2 mm under R=－1. The compressive residual stress induced by SP was reduced due to relaxation. Compared to the results of R=0 and 0.4, the size of semi-circular slit that could be rendered harmless by SP was same. The maximum depth of crack which can be rendered harmless by SP was predicted based on fracture mechanics. The estimated values were in good agreement with experimental values.