Abstract: Nickel-based superalloy exhibits excellent high strength and thermal fatigue resistance at 650 °C, and is therefore widely used in the manufacture of elevated temperature components such as turbine blades for aero-engines. Laser-powder bed fusion (L-PBF) is a rapidly developing metal additive manufacturing technology that is gradually becoming an important method for fabricating nickel-based superalloy products. The design and service life of aero-engine turbine blades usually require more than 107 load cycles, therefore, it is crucial to investigate the very high cycle fatigue characteristics of L-PBF nickel-based superalloy at elevated temperature. Internal failure is a typical elevated temperature fatigue failure mode of L-PBF nickel-base superalloy that is currently not well understood. To overcome this problem, firstly, axial fatigue tests with stress ratios of -1 and 0.1 are carried out at 650 ℃, and partial typical internal failure fractures at a stress ratio of 0.1 are selected as research objects. Secondly, scanning electron microscope and ultra depth field microscope are used to observe the 2D and 3D morphology of the fatigue fracture surface to analyze the crack nucleation area and growth path. The results show that, regardless of the presence of defects, the emergence and aggregation of a number of facets are observed in the "Facetted Cracking Area (FCA)", which is a typical internal failure characteristic of L-PBF nickel-based superalloy. Measurements show that the size of the facets leading to cracking is similar to the size of large grains and is related to differences in grain orientation. Therefore, internal failures are divided into two cracking modes: "defect-assisted faceted cracking" and "non-defect-assisted faceted cracking". Thirdly, the FCA with typical internal failure fractures is cut and subjected to electron backscatter diffraction analysis to observe the surface and subsurface crystallographic features of crack nucleation and growth behavior. The results show that microcracks mainly nucleate from large grains with softer grain orientation and then slip and expand along the direction of maximum shear stress, eventually exhibiting a perforated fracture pattern. Fourthly, subsurface microcrack features below the FCA are then observed in detail using focused ion beam milling and imaging, and slip band and dislocation structures in the vicinity of the microcracks are observed using transmission electron microscopy. The results confirm that the fatigue deformation mechanism of facet at 650 °C is mainly controlled by a combination of anti-phase boundary shearing, precipitate bypassing and stacking fault shearing, especially when subjected to stress concentration effects induced by cracks or defects. Finally, combined with the definition of the crack tip stress intensity factor, a crack nucleation life prediction method related to the characteristics of faceted cracks is proposed, and the prediction results are in good agreement with the experimental results.