This paper presents some test data on the fatigue strength of 12mm test specimens made of various materials to find how fatigue strength in rotating bending is reduced by the presence of shrink-fitted member, and how such a shrink-fitted shaft may be made stronger by induction hardening. From the test results following facts were revealed. (1) The fatigue limit of shrink-fitted shaft does not increase in proportion to the tensile strength, and the fatigue limit of the materials of high tensile strength such as Ni-Cr steel largeness of lowering ratio of the fatigue limit in the former case. So not a remarkable effect can be expected. (2) By induction hardening, the fatigue limit of shrink-fitted shaft made of middle carbon steel (C 0.4%) increases up to about 2, 3 times, and becomes stronger than the shaft made of Ni-Cr steel or of 0.5% carbon steel induction hardened. (3) From these investigations, it is preferable to use the medium carbon steel (C 0.4%) for the shrink-fitted shaft in actual machine parts, since the weakening effect on fatigue strength caused by shrink-fitted member is comparably small, and the increasing effect due to induction hardening is considerably large, and moreover the cost becomes cheaper. Additional data are presented to show how fatigue stength of pre-fatigued shaft is increased by induction hardening, and how hardness of shrink-fitted member influences on the fatigue stength of the shaft.