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Titanium-alloy weldments have been extensively used in engineering structures, and accurate estimation of the fatigue life is beneficial for avoiding catastrophic failures in titanium-alloy structures. The weld pore is a common defect in these weldments that significantly influences the fatigue process. In this study, the weld-pore size and depth and its effect on the fatigue life of Ti-6Al-2Zr-1Mo-1V (TA15) alloy weldments are investigated by adopting fatigue tests and fracture observations. The results show that fatigue crack initiations occurred at the weld pores for all specimens. The diameter and depth of all weld pores on fracture surfaces were then measured, and an indicator, P, was proposed, defined as a combination of pore diameter and depth. It was found that all the fatigue cracks initiated from the pore have the smallest P indicator, which suggests that P can be used to judge the location of crack initiation in an individual sample. Moreover, a model was developed based on P to estimate the fatigue life of weldments, considering the effects of weld-pore size and depth. Finally, analogous fatigue tests were carried out for model verification, and results show that the proposed model has a higher accuracy compared with several typical models. The findings of this study can be helpful in estimating the fatigue life and fatigue design of titanium-alloy weldments.