Inconel 718 is one of the most renowned nickel-based superalloys for high-temperature applications, and the mechanical properties of this alloy produced by conventional techniques are well understood [1]. However, many applications demand highly complex geometries, e.g. to optimise the cooling capability of thermally loaded parts. As a result, additive manufacturing (AM) techniques have recently attracted a lot of interest, as they allow for greater design freedom [2]. However, the microstructural features after AM processing are different from those after conventional processing, and this affects the high-temperature mechanical properties such as fatigue. In this study, high-temperature strain-controlled low cycle fatigue tests were conducted on Laser Powder Bed Fusion (LPBF) manufactured IN718 samples to investigate the impact of stress relaxation and strain ratio. A detailed characterization study was performed on fatigue-induced rupture surfaces to analyse the microstructural effect on the mode of crack propagation at three different temperature regimes. Microstructural features were examined and quantified, including melt pool geometry, lack of fusion defects, keyhole porosity, and sub-grain cellular microstructure. The low cycle fatigue behaviour was described with the use of the Manson-Coffin approach, while the defect formation was analysed with volume fraction and maximum defect size. The purpose of this examination and quantification was to correlate the features to LPBF parameters and fatigue properties. Finally, an attempt was made to correlate the defect formation with fatigue resistance and fatigue rupture life against heterogeneous as-built structures. [1] S.H. Kang, Y. Deguchi, K. Yamamoto, K. Ogi, M. Shirai, Solidification process and behavior of alloying elements in Ni-based superalloy inconel718, Mater. Trans. 45 (2004) 2728–2733. [2] T. Huynh, A. Mehta, K. Graydon, J. Woo, S. Park, H. Hyer, L. Zhou, D.D. Imholte, N.E. Woolstenhulme, D.M. Wachs, Y. Sohn, Microstructural Development in Inconel 718 Nickel-Based Superalloy Additively Manufactured by Laser Powder Bed Fusion, Metallogr. Microstruct. Anal. 11 (2022) 88–107.