The authors have carried out rotating bending fatigue tests of 150 mm specimens with force fitted flange having 350mmφ in external diameter and studied on fatigue strength of force fitted members as well as mechanism of fatigue crack origination. The specimens have been made of three kinds of 0. 2% carbon steel, that is, one charge of 50 ton acid open-hearth steel and two charges of 6 ton electric furnace steel. Small specimens of 10 mmφ having simillar shape have been taken from large specimens and tested for comparison. The following facts have been revealed by the tests ; (1) Fatigue strength of 150 mm φ specimens at 10⁸ stress reversals was 8.0-9.5 kg/mm², and fatigue notch factor based upon 125 mmφ smooth surface specimens 2.80-2.00. (2) Fatigue limit of 15mmφ specimens with forge steel, cast steel and cast iron bushes having 35 mmφ in external diameter was 14.0-14.5 kg/mm², and fatigue notch factor based upon 10 mmφsmooth surface specimen 1.92-1.55. (3) The fatigue strength was decreased to 87% where bronze bushs were used, and increased to 106-108% where thin bushes having 25mmφ in external diameter were used. (4) On the surface of fitted part of large specimens subjected to bending stress below the fatigue limit, fine cracks of non propagating nature were observed. The limit of stress at which cracks will originate was presumed about 5 kg/mm² at 2×10⁷ stress reversals.