A definition of the notch factor associated with program fatigue test, is proposed in the present study and some hypothesis are presented as to how to estimate the notch factor from the conventional fatigue tests.The conclusions are as follows: The notch factor may be estimated as the highest value among the values of the notch factor that we can determine, as a function of assigned number of cycles to failure, from two S-N curves, of which one is for the smooth specimen, and the other for the notched specimen. This hypothesis is practically very useful, because we can apply it to all cases of program load without any program fatigue test, and the estimation from this hypothesis has been proved to be on the safe side from the standpoint of machine design.