Recently, various kinds of high tensile strength steels have been used in ship structures, and their fatigue strength has also been investigated. However, there have been very few reserches on plastic fatigue strength of thin plates under alternating tensile and compressive loads. The experiments on plastic fatigue strength were carried out to clarify the strength of several kinds of steels, and effects of strain concentration and of reinforcement. The following specimens were used under alternating tensile and compressive loads. 1) Plain specimen 2) Notched specimen (with a hole) 3) Inserted specimen (inserted with high tensile strength steel around the hole of notched specimen of mild steel) For the investigation, three kinds of high tensile strength steels (σ _B =50 kg/mm², 60 kg/mm² and 80 kg/mm²) and mild steel were provided. In order to prevent specimens from buckling, a special apparatus was attached on specimens. The results of the experiment are summerized in the followings; 1) S-N curves obtained for alternating amplitude of constant loads have short parallel parts to N-axis, and are different from the results of zero-tension tests. 2) For plain specimens, higher tensile strength steel has greater ratio of fatigue strength to tensile strength than that of mild steel. And there are linear relations between stress amplitudes and tensile strength at the same fatigue life. 3) The strength of the notched specimens generally has the same tendency as for plain specimens, but HT-80 has lower strength than HT-60 in the range of long life (N>10⁴). 4) Fatigue strength of the inserted specimens increases proportionaly to the tensile strength of inserted plates and it's breadth. 5) In the case of very higher stress concentration at the root of the notch, it may be expected that the fatigue strength of inserted specimens has nearly the same fatigue strength as that of notched specimens of high tensile strength steel.