Titanium alloys such as Ti-6Al-4V are used worldwide in the field of aerospace, and the applied results are abundant, and the data are accumulated as structural members. On the other hand, commercial pure (CP) titanium which is cheaper than the alloys is widely used in the field of general industry in Japan because it is superior to seawater corrosion resistance and biocompatibility. However, CP titanium has anisotropy to consist of only hcp crystal structure, therefore, the strength data are insufficient because of the short history as structural materials, and an unidentified part of the mechanical materials properties is remaining. In this study, the effect of mean stress and stress concentration on fatigue strength of CP titanium was evaluated for the application range expansion of CP titanium which has a superior characteristic as structural members in the field of marine. As the result, the following conclusions were provided. Endurance limit of longitudinal (L) direction was 80-84% of endurance limit of transverse (T) direction on smooth specimens, on the other hand, no significant difference was found in endurance limit of both directions on notched specimens. It is necessary to apply at least S a -0.5S u line to design the safe side in CP titanium. Around K t =2, the notch sensitivity factor of CP titanium in T-direction is larger than that of carbon steel and it in L-direction is smaller than that of carbon steel. On the other hand, in K t =3.77, it in L-direction is at the same level as that of S15C or S35C and it in T-direction is at the same level as that of S45C. In the design of the welding structure composed by CP Grade2 titanium which is the nearest to mechanical strength of ship's classifications mild steel, it seems that the design fatigue strength of the steel welding structure is applicable.

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