The　dynamic　metallurgical　characteristics　of　the　selective　laser　melting　(SLM)　process　offer　fabricated　materials　with　non-equilibrium　microstructures　compared　to　their　cast　and　wrought　counterparts.　To　date,　few　studies　on　the　precipitation　kinetics　of　SLM　processed　heat-treatable　alloys　have　been　reported,　despite　the　importance　of　obtaining　such　detailed　knowledge　for　optimizing　the　mechanical　properties.　In　this　study,　for　the　first　time,　the　precipitation　behavior　of　an　SLM　fabricated　Al-Mn-Sc　alloy　was　systematically　investigated　over　the　temperature　range　of　300-450　degrees　C.　The　combination　of　in-situ　synchrotron-based　ultra-small　angle　X-ray　scattering　(USAXS),　small　angle　X-ray　scattering　(SAXS)　and　X-ray　diffraction　(XRD)　revealed　the　continuous　evolution　of　Al6Mn　and　Al3Sc　precipitates　upon　isothermal　heating　in　both　precipitate　structure　and　morphology,　which　was　confirmed　by　ex-situ　transmission　electron　microscopy　(TEM)　studies.　A　pseudo-delay　nucleation　and　growth　phenomenon　of　the　Al3Sc　precipitates　was　observed　for　the　SLM　fabricated　Al-Mn-Sc　alloy.　This　phenomenon　was　attributed　to　the　pre-formed　Sc　clusters　in　the　as-fabricated　condition　due　to　the　intrinsic　heat　treatment　effect　induced　by　the　unique　layer-by-layer　building　nature　of　SLM.　The　growth　kinetics　for　the　Al6Mn　and　Al3Sc　precipitates　were　established　based　on　the　in-situ　X-ray　studies,　with　the　respective　activation　energies　determined　to　be　(74　+/-　4)　kJ/mol　and　(63　+/-　9)　kJ/mol.　The　role　of　the　precipitate　evolution　on　the　final　mechanical　properties　was　evaluated　by　tensile　testing,　and　an　observed　discontinuous　yielding　phenomenon　was　effectively　alleviated　with　increased　aging　temperatures.　(C)　2020　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.