Additive manufacturing techniques have made AM Ti-6Al-4V parts a reality in many industries. However, despite the optimism, their poor fatigue performance especially in high cycle regime is the major hurdle for the industry accepting it as mainstream. One of the reasons owes to the widely distributed internal defects inherent to the AM process, which create a hotbed for fatigue crack initiation. Available investigations on lack of fusions, regarded as the most detrimental defects, are very limited. Regarding this, we conducted finite element analysis to evaluate the fatigue performance of Ti-6Al-4V alloys with an individual lack-of-fusion defect. Three different lack-of-fusion defects, directly scanned from Selective Laser Melting Ti-6Al-4V coupons using Micro-Computed Tomography with different geometry features, have been numerically analyzed. We compare the mechanical results (e.g., stress, strain, and elastic stress concentration factors) of the lack-of-fusion defects to the results of gas-entrapped pores, which share the same height and the same volume, to reveal the detriment of lack-of-fusion defects. Furthermore, we conduct a parametric study on lack-of-fusion defects orientation and size, as well as the aspect ratios. The results provide a better understanding of the mechanical behavior of the lack-of-fusion defects in additive manufactured Ti-6Al-4V alloys, paving the way for further research of additive manufactured metallic alloys.