Abstract: The high cycle fatigue properties of a submerged arc welding joint of one-to-one simulation 25Cr2Ni2MoV specimen used in 1 000 MW nuclear power low pressure turbine rotors were studied. Most of the interior crack sources were porosities, and different zones on the fractured surface can be distinguished under scanning electron microscope. The existing expression on stress intensity factor of crack initiation is well suited to the inclusion. Considering the particularity of the elliptical shape of crack source, the sharp influencing factor for the crack source was introduced to revise the expression. Within the new expression, an approach to predict the threshold was established, by testing high cycle fatigue properties and observing the fracture surface. For the studied welding joint, white-light interfering profilometer was used to measure the area of the crack initiation zone, and logarithm fitting method was applied to get the sharp influencing factor. The predicted threshold value of the welding joint was found in a good agreement with the experiment result.