With the model-semi-built type crankshaft, the diameter of the crank pin is 59mm, being used as a test piece, the fatigue tests under repeated bending were carried out to examine the fatigue strength of the crank pin, which had fillet at the edge. The conclusion may be summarized as follows: (1) The notch factor obtained for the model semi-built type crankshaft is of far larger value than that for a circular shaft with an identical fillet. (2) By changing the radius of the fillet, the notch factor against the fillet was sought. As a result, it was found that within the range where the value of ρ/ d , the ratio between the fillet radius ρ and diameter of the crank pin d , was smaller than 0.05, the notch factor decreased proportionately with the growth of the value of ρ/ d , while, when the value of ρ/ d was larger than 0.05 the notch factor would remain constant. (3) The notch factor for the model with an annular V groove inside the crank arm is larger than that for the model with fillet in the form of a quarter of a circle. (4) In the fatigue test of the model, the crack occured at the fillet part of the end of the crank pin, not at the shrink-fit part of the crank journal.