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This study reports the comprehensive effects of building direction and scanning speed on the fatigue performance of 304L austenitic stainless steel (SS) fabricated by selective laser melting (SLM) through a series of detailed microstructural characterizations, including optical microscopy (OM), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), acoustic emission (AE), and thermal imager (TI). The fatigue tests were performed at a single stress level (90% of yield strength). The correlation analyses between fatigue life and the factors of surface roughness, porosity, and residual stress were performed to elucidate the comprehensive influence of these factors on fatigue lifetime. The combination of fractography analysis, AE signals, and TI signals were used to reveal the underlying microstructural mechanisms during cyclic deformation. Direct evidence is …